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CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Statement of Comprehensive Income [Abstract]        
Net income $ 268.8 $ 237.8 $ 458.0 $ 436.9
Other comprehensive income (loss), net:        
Unrealized gain (loss) on AFS securities, net of tax effect of $(2.0), $(6.0), $23.1, and $(25.3), respectively 5.9 9.7 (68.3) 65.4
Unrealized gain (loss) on SERP, net of tax effect of $—, $0.0, $0.1, and $0.0, respectively 0.0 0.1 (0.3) 0.1
Unrealized (loss) gain on junior subordinated debt, net of tax effect of $0.3, $1.3, $0.0, and $0.9, respectively (0.9) (4.1) 0.1 (3.0)
Reclassification adjustment for gain on sale of AFS securities included in income, net of tax effect of $0.2, $2.9, $13.0, and $3.5, respectively (0.7) (8.8) (38.5) (10.4)
Net other comprehensive income (loss) 4.3 (3.1) (107.0) 52.1
Comprehensive income attributable to noncontrolling interest 7.1 7.4 14.2 7.4
Comprehensive income attributable to Western Alliance $ 266.0 $ 227.3 $ 336.8 $ 481.6