Investment Securities - Additional Information (Detail) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2025 USD ($) security	Jun. 30, 2024 USD ($)	Jun. 30, 2025 USD ($) security	Jun. 30, 2024 USD ($)	Dec. 31, 2024 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	$ 122.0		$ 122.0		$ 117.0
Unrealized gains (losses) on assets measured at fair value, net	(1.3)	$ (1.2)	(0.1)	$ 2.7
Fair value	$ 16,898.0		$ 16,898.0		$ 13,468.0
Number of AFS debt securities in an unrealized loss position \| security	680		680		796
Charge-offs	$ 0.0	0.0	$ 0.0	0.0
Accrued interest receivable	5.0		5.0		$ 5.0
Carrying value of debt securities sold	2,400.0	329.0	2,800.0	1,700.0
Net gain on AFS securities	11.8	2.3	13.9	1.4
Gross gains	11.8	2.7	13.9	3.6
Net gain (loss) recognized		$ 2.3		$ 1.4
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Fair value	5,504.0		5,504.0		4,383.0
U.S. Treasury securities \| Interest rate contracts
Debt Securities, Available-for-sale [Line Items]
Gross gains			7.7
Asset Pledged as Collateral
Debt Securities, Available-for-sale [Line Items]
Fair value	$ 4,900.0		$ 4,900.0		$ 4,000.0

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

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