Parent Company Financial Information - Parent Company Condensed Income Statement (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Condensed Financial Statements, Captions [Line Items]
Non-interest income (loss)	$ 543.2	$ 280.7	$ 324.6
Interest expense	1,922.2	1,696.4	475.5
Non-interest expense	2,025.0	1,623.4	1,156.7
Income tax benefit	(203.5)	(211.2)	(258.8)
Net income	787.7	722.4	1,057.3
Dividends on preferred stock	12.8	12.8	12.8
Net income available to common stockholders	774.9	709.6	1,044.5
WAL
Condensed Financial Statements, Captions [Line Items]
Dividends from subsidiaries	240.0	330.0	261.8
Interest income	2.7	2.9	3.8
Non-interest income (loss)	22.5	1.5	(0.9)
Total income	265.2	334.4	264.7
Interest expense	25.7	25.4	22.6
Non-interest expense	27.4	29.3	26.5
Total expense	53.1	54.7	49.1
Income before income taxes and equity in undistributed earnings of subsidiaries	212.1	279.7	215.6
Income tax benefit	6.1	10.3	11.0
Income before equity in undistributed earnings of subsidiaries	218.2	290.0	226.6
Equity in undistributed earnings of subsidiaries	569.5	432.4	830.7
Net income	$ 787.7	$ 722.4	$ 1,057.3