Derivative Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024	Mar. 31, 2025	Dec. 31, 2024
Derivative
Credit Exposure on Derivatives	$ 0.2		$ 0.2			$ 0.5
Aggregate fair value of all derivative instruments with credit risk-related contingent features in a liability position	286.5		286.5			255.7
Derivative, Notional Amount	3,815.4		3,815.4			3,751.4
Approximate amount of net deferred gains on derivative instruments expected to be amortized during the next twelve months			5.2
Component of Derivative Gain (Loss) Excluded from the Assessment of Hedge Effectiveness	(2.3)	$ 11.2	24.9	$ 1.3
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			28 years
Net Gain (Loss) on Derivatives	2.1	0.4	$ (3.2)	2.0
Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	3,502.8		3,502.8			3,445.5
Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	312.6		312.6			305.9
Net Gain (Loss) on Derivatives	3.1	1.2	(4.0)	8.9
Cash Flow Hedging | Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	2,733.3		2,733.3			2,709.1
Interest Rate Swaps | Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	125.9		125.9			125.9
Interest Rate Swaps | Cash Flow Hedging | Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	136.3		136.3			139.1
Interest Rate Swaps | Cash Flow Hedging | Designated as Hedging Instrument | Investment Income [Member]
Derivative
Net Gain (Loss) on Derivatives	2.9	4.8	5.5	10.8
Interest Rate Swaps | Cash Flow Hedging | Designated as Hedging Instrument | Net Investment Gain (Loss)
Derivative
Net Gain (Loss) on Derivatives	(0.4)	0.0	0.0	0.0
Interest Rate Swaps | Fair Value Hedging | Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	769.5		769.5			736.4
Forwards | Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	47.5		47.5			51.1
Forwards | Cash Flow Hedging | Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	2,597.0		2,597.0			2,570.0
Forwards | Cash Flow Hedging | Designated as Hedging Instrument | Investment Income [Member]
Derivative
Net Gain (Loss) on Derivatives	(0.4)	(0.2)	(0.8)	(0.4)
Forwards | Cash Flow Hedging | Designated as Hedging Instrument | Net Investment Gain (Loss)
Derivative
Net Gain (Loss) on Derivatives	0.0	0.0	0.0	0.0
Total Return Swap | Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	139.2		139.2			128.9
Net Gain (Loss) on Derivatives	(11.3)	(0.8)	(7.0)	(8.7)
Total Return Swap | Not Designated as Hedging Instrument | Investment Income [Member]
Derivative
Net Gain (Loss) on Derivatives	0.8	0.0	0.9	0.0
Total Return Swap | Not Designated as Hedging Instrument | Net Investment Gain (Loss)
Derivative
Net Gain (Loss) on Derivatives	6.0	0.0	2.8	0.0
Foreign Exchange Contracts | Not Designated as Hedging Instrument
Derivative
Net Gain (Loss) on Derivatives	(3.9)	0.4	(5.9)	2.0
Foreign Exchange Contracts | Cash Flow Hedging | Designated as Hedging Instrument | Investment Income [Member]
Derivative
Net Gain (Loss) on Derivatives	0.3	0.1	(0.4)	(0.4)
Foreign Exchange Contracts | Cash Flow Hedging | Designated as Hedging Instrument | Net Investment Gain (Loss)
Derivative
Net Gain (Loss) on Derivatives	0.1	(0.2)	0.1	(0.2)
Foreign Exchange Contracts | Fair Value Hedging | Designated as Hedging Instrument | Investment Income [Member]
Derivative
Net Gain (Loss) on Derivatives	2.9	4.2	(2.3)	7.3
Foreign Exchange Contracts | Fair Value Hedging | Designated as Hedging Instrument | Net Investment Gain (Loss)
Derivative
Net Gain (Loss) on Derivatives	(50.6)	2.0	(72.1)	19.6
Embedded Derivative in Modified Coinsurance Arrangement | Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	0.0		0.0			0.0
Net Gain (Loss) on Derivatives	1.0	$ 0.8	(0.9)	$ 6.9
Receive Fixed/Pay Fixed | Interest Rate Swaps | Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount						125.9
Receive Fixed/Pay Fixed | Interest Rate Swaps | Cash Flow Hedging | Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	136.3		136.3			139.1
Receive Fixed/Pay Fixed | Interest Rate Swaps | Fair Value Hedging | Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	769.5		769.5			736.4
Credit Spreads and Interest Rate Volatility | Total Return Swap | Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	$ 0.0		$ 0.0		$ 700.0	$ 0.0