Loans, Leases and Allowance for Credit Losses - Amortized Cost Basis of Loans Modified (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 107	$ 89	$ 160	$ 223
% of Total Class of Financing Receivable	0.20%	0.20%	0.30%	0.40%
Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 0	$ 0	$ 0	$ 49
Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	2	0	2
Payment Delay
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	107	87	160	172
Tech & innovation
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		$ 2	$ 18	$ 31
% of Total Class of Financing Receivable		0.10%	0.50%	1.00%
Tech & innovation \| Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		$ 0	$ 0	$ 0
Tech & innovation \| Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		2	0	2
Tech & innovation \| Payment Delay
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		0	18	29
Other commercial and industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 57	$ 87	$ 58	$ 95
% of Total Class of Financing Receivable	0.40%	1.00%	0.40%	1.00%
Other commercial and industrial \| Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 0	$ 0	$ 0	$ 8
Other commercial and industrial \| Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0	0	0
Other commercial and industrial \| Payment Delay
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	57	$ 87	58	87
Other CRE - non-owner occupied
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 50		$ 50	$ 56
% of Total Class of Financing Receivable	0.70%		0.70%	0.90%
Other CRE - non-owner occupied \| Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 0		$ 0	$ 0
Other CRE - non-owner occupied \| Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0		0	0
Other CRE - non-owner occupied \| Payment Delay
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 50		50	56
Construction and land development
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis			$ 34	$ 41
% of Total Class of Financing Receivable			0.80%	0.90%
Construction and land development \| Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis			$ 0	$ 41
Construction and land development \| Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis			0	0
Construction and land development \| Payment Delay
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis			$ 34	$ 0

WESTERN ALLIANCE BANCORPORATION 10-Q