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Investment Securities - Additional Information (Detail) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026 USD ($) security	Jun. 30, 2025 USD ($)	Jun. 30, 2026 USD ($) security	Jun. 30, 2025 USD ($)	Dec. 31, 2025 USD ($) security	Mar. 31, 2025 USD ($)	Dec. 31, 2024 USD ($)
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	$ 80.0		$ 80.0		$ 79.0
Unrealized gains (losses) on assets measured at fair value, net	0.4	$ (1.3)	(0.4)	$ (0.1)
Fair value	$ 18,880.0		$ 18,880.0		$ 18,788.0
Number of AFS debt securities in an unrealized loss position \| security	851		851		568
Allowance for credit loss	$ 0.0	0.3	$ 0.0	0.3		$ 0.2	$ 0.4
Accrued interest receivable, AFS	141.0		141.0		$ 135.0
Accrued interest receivable, HTM	6.0		6.0		5.0
Carrying value of debt securities sold	583.0	2,400.0	7,100.0	2,800.0
Net gain on AFS securities	1.4	11.8	51.9	13.9
Gross gains	1.4	11.8	59.0	13.9
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Fair value	5,494.0		5,494.0		5,970.0
U.S. Treasury securities \| Interest rate contracts
Debt Securities, Available-for-sale [Line Items]
Gross gains		$ 7.7	18.7	$ 7.7
Asset Pledged as Collateral
Debt Securities, Available-for-sale [Line Items]
Fair value	$ 8,600.0		$ 8,600.0		$ 9,100.0