Loans, Leases and Allowance for Credit Losses - Contractual Aging of Loan Portfolio by Segment (Detail) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	$ 59,142	$ 58,677
Total Nonaccrual	492	500
Mortgage finance
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	7,105	7,271
Total Nonaccrual	0	0
Municipal & nonprofit
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	1,683	1,648
Total Nonaccrual	4	4
Tech & innovation
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	4,280	4,128
Total Nonaccrual	22	20
Equity fund resources
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	1,261	1,233
Total Nonaccrual	1	1
Other commercial and industrial
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	14,132	13,789
Total Nonaccrual	98	120
CRE - owner occupied
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	1,563	1,533
Total Nonaccrual	3	3
Hotel franchise finance
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	4,331	4,185
Total Nonaccrual	0	0
Other CRE - non-owner occupied
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	6,334	6,455
Total Nonaccrual	263	228
Residential
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	13,547	13,403
Total Nonaccrual	13	12
Residential EBO
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	792	828
Total Nonaccrual	0	0
Construction and land development
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	3,961	4,043
Total Nonaccrual	85	109
Other
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	153	161
Total Nonaccrual	3	3
Current
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	58,055	57,568
Current \| Mortgage finance
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	7,105	7,271
Current \| Municipal & nonprofit
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	1,676	1,641
Current \| Tech & innovation
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	4,210	4,102
Current \| Equity fund resources
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	1,260	1,232
Current \| Other commercial and industrial
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	14,025	13,654
Current \| CRE - owner occupied
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	1,557	1,530
Current \| Hotel franchise finance
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	4,331	4,185
Current \| Other CRE - non-owner occupied
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	6,068	6,226
Current \| Residential
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	13,399	13,259
Current \| Residential EBO
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	409	393
Current \| Construction and land development
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	3,866	3,920
Current \| Other
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	149	155
30-59 Days Past Due
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	182	172
30-59 Days Past Due \| Mortgage finance
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	0	0
30-59 Days Past Due \| Municipal & nonprofit
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	0	0
30-59 Days Past Due \| Tech & innovation
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	31	3
30-59 Days Past Due \| Equity fund resources
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	0	0
30-59 Days Past Due \| Other commercial and industrial
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	8	12
30-59 Days Past Due \| CRE - owner occupied
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	1	0
30-59 Days Past Due \| Hotel franchise finance
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	0	0
30-59 Days Past Due \| Other CRE - non-owner occupied
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	3	1
30-59 Days Past Due \| Residential
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	66	55
30-59 Days Past Due \| Residential EBO
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	62	94
30-59 Days Past Due \| Construction and land development
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	10	5
30-59 Days Past Due \| Other
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	1	2
60-89 Days Past Due
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	69	81
60-89 Days Past Due \| Mortgage finance
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	0	0
60-89 Days Past Due \| Municipal & nonprofit
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	0	0
60-89 Days Past Due \| Tech & innovation
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	17	0
60-89 Days Past Due \| Equity fund resources
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	0	0
60-89 Days Past Due \| Other commercial and industrial
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	1	3
60-89 Days Past Due \| CRE - owner occupied
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	2	0
60-89 Days Past Due \| Hotel franchise finance
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	0	0
60-89 Days Past Due \| Other CRE - non-owner occupied
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	0	0
60-89 Days Past Due \| Residential
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	17	26
60-89 Days Past Due \| Residential EBO
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	32	51
60-89 Days Past Due \| Construction and land development
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	0	0
60-89 Days Past Due \| Other
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	0	1
Over 90 days Past Due
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	344	356
Over 90 days Past Due \| Mortgage finance
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	0	0
Over 90 days Past Due \| Municipal & nonprofit
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	3	3
Over 90 days Past Due \| Tech & innovation
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	0	3
Over 90 days Past Due \| Equity fund resources
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	0	0
Over 90 days Past Due \| Other commercial and industrial
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	0	0
Over 90 days Past Due \| CRE - owner occupied
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	0	0
Over 90 days Past Due \| Hotel franchise finance
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	0	0
Over 90 days Past Due \| Other CRE - non-owner occupied
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	0	0
Over 90 days Past Due \| Residential
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	52	51
Over 90 days Past Due \| Residential EBO
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	289	290
Over 90 days Past Due \| Construction and land development
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	0	9
Over 90 days Past Due \| Other
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	0	0
Total Past Due
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	595	609
Total Past Due \| Mortgage finance
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	0	0
Total Past Due \| Municipal & nonprofit
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	3	3
Total Past Due \| Tech & innovation
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	48	6
Total Past Due \| Equity fund resources
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	0	0
Total Past Due \| Other commercial and industrial
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	9	15
Total Past Due \| CRE - owner occupied
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	3	0
Total Past Due \| Hotel franchise finance
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	0	0
Total Past Due \| Other CRE - non-owner occupied
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	3	1
Total Past Due \| Residential
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	135	132
Total Past Due \| Residential EBO
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	383	435
Total Past Due \| Construction and land development
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	10	14
Total Past Due \| Other
Financing Receivable, Nonaccrual [Line Items]
Loans HFI, net of deferred fees and costs	$ 1	$ 3

WESTERN ALLIANCE BANCORPORATION 10-Q