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Fair value of derivative asset subject to concentration of credit or market risk or both.

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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		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
Derivative
Credit Exposure on Derivatives	$ 0.3		$ 0.3		$ 1.6
Aggregate fair value of all derivative instruments with credit risk-related contingent features in a liability position	130.2		130.2		116.2
Derivative, Notional Amount	3,665.5		3,665.5		2,983.7
Approximate amount of net deferred gains on derivative instruments expected to be amortized during the next twelve months			9.2
Component of Derivative Gain (Loss) Excluded from the Assessment of Hedge Effectiveness	8.2	$ (19.4)	9.5	$ (26.6)
Discontinued Hedge Accounting Due to Instrument No Longer Qualifying as Fair Value Hedge	0.0	$ 0.0	$ 0.0	$ 0.0
Maximum Length of Time Hedged in Cash Flow Hedge			39 years
Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	3,358.7		$ 3,358.7		2,697.0
Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	306.8		306.8		286.7
Cash Flow Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	2,665.5		2,665.5		2,054.5
Interest Rate Swaps \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	125.9		125.9		132.0
Interest Rate Swaps \| Cash Flow Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	142.5		142.5		149.5
Interest Rate Swaps \| Fair Value Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	693.2		693.2		642.5
Forwards \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	54.6		54.6		52.5
Forwards \| Cash Flow Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	2,523.0		2,523.0		1,905.0
Total Return Swap \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	126.3		126.3		102.2
Embedded Derivative in Modified Coinsurance Arrangement \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	0.0		0.0		0.0
Receive Fixed/Pay Fixed \| Interest Rate Swaps \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	125.9		125.9		132.0
Receive Fixed/Pay Fixed \| Interest Rate Swaps \| Cash Flow Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	142.5		142.5		149.5
Receive Fixed/Pay Fixed \| Interest Rate Swaps \| Fair Value Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	$ 693.2		$ 693.2		$ 642.5