Derivative Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025	Dec. 31, 2025	Mar. 31, 2025
Derivative
Aggregate fair value of all derivative instruments with credit risk-related contingent features in a liability position	$ 421.5		$ 421.5		$ 269.7
Derivative, Notional Amount	5,157.0		5,157.0		3,818.2
Approximate amount of net deferred gains on derivative instruments expected to be amortized during the next twelve months			(0.4)
Component of Derivative Gain (Loss) Excluded from the Assessment of Hedge Effectiveness	(16.1)	$ 6.4	(23.5)	$ 31.3
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
Derivative Asset, Fair Value, Gross Asset	69.0		$ 69.0		69.7
Gross Derivative Liability	421.5		421.5		269.7
Gain (Loss) on Investments	(853.0)	128.0	(863.2)	(96.5)
Credit Exposure
Derivative
Credit Exposure on Derivatives	2.2		2.2		0.0
Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	3,268.2		3,268.2		3,498.9
Derivative Asset, Fair Value, Gross Asset	44.6		44.6		51.6
Gross Derivative Liability	344.6		344.6		253.6
Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	1,888.8		1,888.8		319.3
Derivative Asset, Fair Value, Gross Asset	24.4		24.4		18.1
Gross Derivative Liability	76.9		76.9		16.1
Gain (Loss) on Investments	(69.9)	3.3	(69.9)	(0.7)
Cash Flow Hedging | Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	2,420.2		2,420.2		2,730.3
Derivative Asset, Fair Value, Gross Asset	11.2		11.2		16.6
Gross Derivative Liability	318.1		318.1		224.6
Interest Rate Swaps | Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	205.8		205.8
Derivative Asset, Fair Value, Gross Asset	4.7		4.7
Gross Derivative Liability	0.0		0.0
Gain (Loss) on Investments	4.7	0.0	4.7	0.0
Interest Rate Swaps | Cash Flow Hedging
Derivative
Gain (Loss) on Investments	(17.4)	39.2	(17.4)	38.9
Interest Rate Swaps | Cash Flow Hedging | Designated as Hedging Instrument
Derivative
Gain (Loss) on Investments	0.0	2.2	0.0	2.2
Total Return Swap | Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	722.3		722.3		154.8
Derivative Asset, Fair Value, Gross Asset	0.0		0.0		0.0
Gross Derivative Liability	20.6		20.6		0.0
Gain (Loss) on Investments	(27.2)	0.0	(27.2)	2.8
Foreign Exchange Contracts | Not Designated as Hedging Instrument
Derivative
Gain (Loss) on Investments	0.9	0.0	3.6	(5.9)
Foreign Exchange Contracts | Cash Flow Hedging
Derivative
Gain (Loss) on Investments	0.9	(0.9)	(2.3)	(1.0)
Foreign Exchange Contracts | Cash Flow Hedging | Designated as Hedging Instrument
Derivative
Gain (Loss) on Investments	(1.0)	0.8	1.0	0.9
Foreign Exchange Contracts | Fair Value Hedging
Derivative
Gain (Loss) on Investments	(8.9)	(2.3)	(24.4)	69.8
Foreign Exchange Contracts | Fair Value Hedging | Designated as Hedging Instrument
Derivative
Gain (Loss) on Investments	8.9	2.3	24.4	(69.8)
Embedded Derivative in Modified Coinsurance Arrangement | Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	0.0		0.0		0.0
Derivative Asset, Fair Value, Gross Asset	18.7		18.7		17.8
Gross Derivative Liability	0.0		0.0		0.0
Gain (Loss) on Investments	3.6	3.3	0.9	2.4
Foreign Exchange Forward | Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	45.8		45.8		48.9
Derivative Asset, Fair Value, Gross Asset	1.0		1.0		0.1
Gross Derivative Liability	0.4		0.4		1.0
Forward Benchmark Interest Rate Lock | Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	832.0		832.0
Derivative Asset, Fair Value, Gross Asset	0.0		0.0
Gross Derivative Liability	45.2		45.2
Gain (Loss) on Investments	(51.9)	0.0	(51.9)	0.0
Forward Benchmark Interest Rate Lock | Cash Flow Hedging
Derivative
Gain (Loss) on Investments	(24.4)	0.5	(24.4)	0.5
Forward Benchmark Interest Rate Lock | Cash Flow Hedging | Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	2,311.0		2,311.0		2,603.0
Derivative Asset, Fair Value, Gross Asset	0.0		0.0		2.3
Gross Derivative Liability	317.2		317.2		222.9
Gain (Loss) on Investments	(10.0)	$ (3.1)	(10.0)	$ (3.1)
Receive Fixed/Pay Fixed | Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	82.9		82.9		115.6
Derivative Asset, Fair Value, Gross Asset	0.0		0.0		0.2
Gross Derivative Liability	10.7		10.7		15.1
Receive Fixed/Pay Fixed | Cash Flow Hedging | Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	109.2		109.2		127.3
Derivative Asset, Fair Value, Gross Asset	11.2		11.2		14.3
Gross Derivative Liability	0.9		0.9		1.7
Receive Fixed/Pay Fixed | Fair Value Hedging | Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	848.0		848.0		768.6
Derivative Asset, Fair Value, Gross Asset	33.4		33.4		35.0
Gross Derivative Liability	26.5		26.5		29.0
Receive Fixed/Pay Fixed | Interest Rate Swaps | Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	82.9		82.9		115.6
Receive Fixed/Pay Fixed | Interest Rate Swaps | Cash Flow Hedging | Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	109.2		109.2		127.3
Receive Fixed/Pay Fixed | Interest Rate Swaps | Fair Value Hedging | Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	848.0		848.0		768.6
Credit Spreads and Interest Rate Volatility | Total Return Swap | Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount						$ 700.0
Fair Value Volatility | Total Return Swap | Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	172.5		172.5		154.8
Consideration Payable | Total Return Swap | Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	549.8		549.8		0.0
Consideration Payable | Forward Benchmark Interest Rate Lock | Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	832.0		832.0		0.0
Interest Rate Risk | Total Return Swap | Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	$ 205.8		$ 205.8		$ 0.0