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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, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Derivative
Aggregate fair value of all derivative instruments with credit risk-related contingent features in a liability position	$ 212.7		$ 212.7		$ 74.0
Derivative, Notional Amount	2,324.2		2,324.2		1,753.9
Approximate amount of net deferred gains on derivative instruments expected to be amortized during the next twelve months			21.4
Component of Derivative Gain (Loss) Excluded from the Assessment of Hedge Effectiveness	(19.4)	$ 0.2	(26.6)	$ 13.9
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			40 years 3 months
Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	2,050.0		$ 2,050.0		1,490.7
Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	274.2		274.2		263.2
Cash Flow Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	1,446.4		1,446.4		932.9
Interest Rate Swaps \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	132.0		132.0		132.0
Interest Rate Swaps \| Cash Flow Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	156.4		156.4		168.9
Interest Rate Swaps \| Fair Value Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	603.6		603.6		557.8
Forwards \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	53.2		53.2		54.3
Forwards \| Cash Flow Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	1,290.0		1,290.0		764.0
Total Return Swap \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	89.0		89.0		76.9
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	132.0		132.0		132.0
Receive Fixed/Pay Fixed \| Interest Rate Swaps \| Cash Flow Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	156.4		156.4		168.9
Receive Fixed/Pay Fixed \| Interest Rate Swaps \| Fair Value Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	603.6		603.6		557.8
Credit Exposure
Derivative
Credit Exposure on Derivatives	$ 0.0		$ 0.0		$ 1.7