Investment Securities - Investment Securities by Credit Rating Type (Detail) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]
Investment securities - amortized cost	$ 1,584	$ 1,526
Fair value	18,788	13,468
Investment securities - equity	79	117
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Fair value	5,970	4,383
Residential MBS issued by GSEs and GNMA
Debt Securities, Available-for-sale [Line Items]
Fair value	7,230	5,831
CLO
Debt Securities, Available-for-sale [Line Items]
Fair value	2,747	570
Private label residential MBS
Debt Securities, Available-for-sale [Line Items]
Investment securities - amortized cost	165	176
Fair value	1,039	947
Tax-exempt
Debt Securities, Available-for-sale [Line Items]
Investment securities - amortized cost	1,419	1,350
Fair value	802	845
Commercial MBS issued by GSEs and GNMA
Debt Securities, Available-for-sale [Line Items]
Fair value	635	437
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Fair value	297	386
Other
Debt Securities, Available-for-sale [Line Items]
Fair value	68	69
Preferred Stock
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	52	91
CRA investments
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	27	26
AAA
Debt Securities, Available-for-sale [Line Items]
Investment securities - amortized cost	0	0
Fair value	1,760	980
Investment securities - equity	0	0
AAA \| U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Fair value	0	0
AAA \| Residential MBS issued by GSEs and GNMA
Debt Securities, Available-for-sale [Line Items]
Fair value	0	0
AAA \| CLO
Debt Securities, Available-for-sale [Line Items]
Fair value	739	50
AAA \| Private label residential MBS
Debt Securities, Available-for-sale [Line Items]
Investment securities - amortized cost	0	0
Fair value	1,012	921
AAA \| Tax-exempt
Debt Securities, Available-for-sale [Line Items]
Investment securities - amortized cost	0	0
Fair value	8	9
AAA \| Commercial MBS issued by GSEs and GNMA
Debt Securities, Available-for-sale [Line Items]
Fair value	1	0
AAA \| Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Fair value	0	0
AAA \| Other
Debt Securities, Available-for-sale [Line Items]
Fair value	0	0
AAA \| Preferred Stock
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	0	0
AAA \| CRA investments
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	0	0
Split-rated AAA/AA+
Debt Securities, Available-for-sale [Line Items]
Investment securities - amortized cost	0	0
Fair value	0	10,671
Investment securities - equity	0	26
Split-rated AAA/AA+ \| U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Fair value	0	4,383
Split-rated AAA/AA+ \| Residential MBS issued by GSEs and GNMA
Debt Securities, Available-for-sale [Line Items]
Fair value	0	5,831
Split-rated AAA/AA+ \| CLO
Debt Securities, Available-for-sale [Line Items]
Fair value	0	0
Split-rated AAA/AA+ \| Private label residential MBS
Debt Securities, Available-for-sale [Line Items]
Investment securities - amortized cost	0	0
Fair value	0	0
Split-rated AAA/AA+ \| Tax-exempt
Debt Securities, Available-for-sale [Line Items]
Investment securities - amortized cost	0	0
Fair value	0	19
Split-rated AAA/AA+ \| Commercial MBS issued by GSEs and GNMA
Debt Securities, Available-for-sale [Line Items]
Fair value	0	437
Split-rated AAA/AA+ \| Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Fair value	0	0
Split-rated AAA/AA+ \| Other
Debt Securities, Available-for-sale [Line Items]
Fair value	0	1
Split-rated AAA/AA+ \| Preferred Stock
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	0	0
Split-rated AAA/AA+ \| CRA investments
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	0	26
AA+ to AA-
Debt Securities, Available-for-sale [Line Items]
Investment securities - amortized cost	0	0
Fair value	16,090	847
Investment securities - equity	27	0
AA+ to AA- \| U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Fair value	5,970	0
AA+ to AA- \| Residential MBS issued by GSEs and GNMA
Debt Securities, Available-for-sale [Line Items]
Fair value	7,230	0
AA+ to AA- \| CLO
Debt Securities, Available-for-sale [Line Items]
Fair value	1,857	465
AA+ to AA- \| Private label residential MBS
Debt Securities, Available-for-sale [Line Items]
Investment securities - amortized cost	0	0
Fair value	26	26
AA+ to AA- \| Tax-exempt
Debt Securities, Available-for-sale [Line Items]
Investment securities - amortized cost	0	0
Fair value	362	348
AA+ to AA- \| Commercial MBS issued by GSEs and GNMA
Debt Securities, Available-for-sale [Line Items]
Fair value	634	0
AA+ to AA- \| Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Fair value	0	0
AA+ to AA- \| Other
Debt Securities, Available-for-sale [Line Items]
Fair value	11	8
AA+ to AA- \| Preferred Stock
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	0	0
AA+ to AA- \| CRA investments
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	27	0
A+ to A-
Debt Securities, Available-for-sale [Line Items]
Investment securities - amortized cost	0	0
Fair value	593	510
Investment securities - equity	0	0
A+ to A- \| U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Fair value	0	0
A+ to A- \| Residential MBS issued by GSEs and GNMA
Debt Securities, Available-for-sale [Line Items]
Fair value	0	0
A+ to A- \| CLO
Debt Securities, Available-for-sale [Line Items]
Fair value	151	55
A+ to A- \| Private label residential MBS
Debt Securities, Available-for-sale [Line Items]
Investment securities - amortized cost	0	0
Fair value	0	0
A+ to A- \| Tax-exempt
Debt Securities, Available-for-sale [Line Items]
Investment securities - amortized cost	0	0
Fair value	361	375
A+ to A- \| Commercial MBS issued by GSEs and GNMA
Debt Securities, Available-for-sale [Line Items]
Fair value	0	0
A+ to A- \| Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Fair value	78	78
A+ to A- \| Other
Debt Securities, Available-for-sale [Line Items]
Fair value	3	2
A+ to A- \| Preferred Stock
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	0	0
A+ to A- \| CRA investments
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	0	0
BBB+ to BBB-
Debt Securities, Available-for-sale [Line Items]
Investment securities - amortized cost	0	0
Fair value	166	266
Investment securities - equity	21	50
BBB+ to BBB- \| U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Fair value	0	0
BBB+ to BBB- \| Residential MBS issued by GSEs and GNMA
Debt Securities, Available-for-sale [Line Items]
Fair value	0	0
BBB+ to BBB- \| CLO
Debt Securities, Available-for-sale [Line Items]
Fair value	0	0
BBB+ to BBB- \| Private label residential MBS
Debt Securities, Available-for-sale [Line Items]
Investment securities - amortized cost	0	0
Fair value	0	0
BBB+ to BBB- \| Tax-exempt
Debt Securities, Available-for-sale [Line Items]
Investment securities - amortized cost	0	0
Fair value	0	0
BBB+ to BBB- \| Commercial MBS issued by GSEs and GNMA
Debt Securities, Available-for-sale [Line Items]
Fair value	0	0
BBB+ to BBB- \| Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Fair value	138	226
BBB+ to BBB- \| Other
Debt Securities, Available-for-sale [Line Items]
Fair value	28	40
BBB+ to BBB- \| Preferred Stock
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	21	50
BBB+ to BBB- \| CRA investments
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	0	0
BB+ and below
Debt Securities, Available-for-sale [Line Items]
Investment securities - amortized cost	0	0
Fair value	92	83
Investment securities - equity	30	29
BB+ and below \| U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Fair value	0	0
BB+ and below \| Residential MBS issued by GSEs and GNMA
Debt Securities, Available-for-sale [Line Items]
Fair value	0	0
BB+ and below \| CLO
Debt Securities, Available-for-sale [Line Items]
Fair value	0	0
BB+ and below \| Private label residential MBS
Debt Securities, Available-for-sale [Line Items]
Investment securities - amortized cost	0	0
Fair value	1	0
BB+ and below \| Tax-exempt
Debt Securities, Available-for-sale [Line Items]
Investment securities - amortized cost	0	0
Fair value	0	0
BB+ and below \| Commercial MBS issued by GSEs and GNMA
Debt Securities, Available-for-sale [Line Items]
Fair value	0	0
BB+ and below \| Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Fair value	81	82
BB+ and below \| Other
Debt Securities, Available-for-sale [Line Items]
Fair value	10	1
BB+ and below \| Preferred Stock
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	30	29
BB+ and below \| CRA investments
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	0	0
Unrated
Debt Securities, Available-for-sale [Line Items]
Investment securities - amortized cost	1,584	1,526
Fair value	87	111
Investment securities - equity	1	12
Unrated \| U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Fair value	0	0
Unrated \| Residential MBS issued by GSEs and GNMA
Debt Securities, Available-for-sale [Line Items]
Fair value	0	0
Unrated \| CLO
Debt Securities, Available-for-sale [Line Items]
Fair value	0	0
Unrated \| Private label residential MBS
Debt Securities, Available-for-sale [Line Items]
Investment securities - amortized cost	165	176
Fair value	0	0
Unrated \| Tax-exempt
Debt Securities, Available-for-sale [Line Items]
Investment securities - amortized cost	1,419	1,350
Fair value	71	94
Unrated \| Commercial MBS issued by GSEs and GNMA
Debt Securities, Available-for-sale [Line Items]
Fair value	0	0
Unrated \| Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Fair value	0	0
Unrated \| Other
Debt Securities, Available-for-sale [Line Items]
Fair value	16	17
Unrated \| Preferred Stock
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	1	12
Unrated \| CRA investments
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	$ 0	$ 0

WESTERN ALLIANCE BANCORPORATION 10-K