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Net gain (loss) relating to components of the gain (loss) on the fair value hedging instrument excluded from the assessment of fair value hedge effectiveness. Recognized in earnings.

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Derivative Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025	Dec. 31, 2025
Derivative
Aggregate fair value of all derivative instruments with credit risk-related contingent features in a liability position	$ 223.0		$ 223.0		$ 269.7
Derivative, Notional Amount	3,718.3		3,718.3		3,818.2
Approximate amount of net deferred gains on derivative instruments expected to be amortized during the next twelve months			0.9
Component of Derivative Gain (Loss) Excluded from the Assessment of Hedge Effectiveness	(12.1)	$ (2.3)	(7.4)	$ 24.9
Discontinued Hedge Accounting Due to Instrument No Longer Qualifying as Fair Value Hedge	0.0	$ 0.0	$ 0.0	$ 0.0
Cash Flow Hedge, Hedging Exposure, Maximum Period			27 years
Credit Exposure
Derivative
Credit Exposure on Derivatives	1.1		$ 1.1		0.0
Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	3,411.7		3,411.7		3,498.9
Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	306.6		306.6		319.3
Cash Flow Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	2,529.6		2,529.6		2,730.3
Interest Rate Swaps \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	89.0		89.0		115.6
Interest Rate Swaps \| Cash Flow Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	112.6		112.6		127.3
Interest Rate Swaps \| Fair Value Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	882.1		882.1		768.6
Total Return Swap \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	171.8		171.8		154.8
Embedded Derivative in Modified Coinsurance Arrangement \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	0.0		0.0		0.0
Foreign Exchange Forward \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	45.8		45.8		48.9
Forward Benchmark Interest Rate Lock \| Cash Flow Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	2,417.0		2,417.0		2,603.0
Receive Fixed/Pay Fixed \| Interest Rate Swaps \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	89.0		89.0		115.6
Receive Fixed/Pay Fixed \| Interest Rate Swaps \| Cash Flow Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	112.6		112.6		127.3
Receive Fixed/Pay Fixed \| Interest Rate Swaps \| Fair Value Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	882.1		882.1		768.6
Credit Spreads and Interest Rate Volatility \| Total Return Swap \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	0.0		0.0		700.0
Fair Value Volatility \| Total Return Swap \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	$ 171.8		$ 171.8		$ 154.8