Investment Securities - Additional Information (Detail) $ in Millions	3 Months Ended
Mar. 31, 2026 USD ($) security	Mar. 31, 2025 USD ($)	Dec. 31, 2025 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	$ 79.0		$ 79.0
Unrealized gains (losses) on assets measured at fair value, net	(0.8)	$ (1.3)
Fair value	$ 18,701.0		$ 18,788.0
Number of AFS debt securities in an unrealized loss position \| security	804		568
Charge-offs	$ 0.0	0.0
Carrying value of debt securities sold	6,500.0	410.0
Net gain on AFS securities	50.5	2.1
Gross gains	57.6	$ 2.1
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Fair value	5,610.0		$ 5,970.0
U.S. Treasury securities \| Interest rate contracts
Debt Securities, Available-for-sale [Line Items]
Gross gains	18.7
Available-for-Sale Securities
Debt Securities, Available-for-sale [Line Items]
Accrued interest receivable	126.0		135.0
Held-to-Maturity Securities
Debt Securities, Available-for-sale [Line Items]
Accrued interest receivable	6.0		5.0
Asset Pledged as Collateral
Debt Securities, Available-for-sale [Line Items]
Fair value	$ 7,600.0		$ 9,100.0

Interest, dividends, rents, ancillary and other revenues earned but not yet received by the entity on its investments.

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