Location and Amounts of Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Derivative
Derivative, Notional Amount	$ 3,769.4	$ 3,818.2
Derivative Instrument by Balance Sheet Location: us-gaap:OtherLiabilities
Derivative
Liability Derivatives Fair Value	261.0	269.7
Derivative Instrument by Balance Sheet Location: us-gaap:OtherLongTermInvestments
Derivative
Asset Derivatives Fair Value	75.5	69.7
Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	3,468.6	3,498.9
Designated as Hedging Instrument | Cash Flow Hedging
Derivative
Derivative, Notional Amount	2,646.0	2,730.3
Designated as Hedging Instrument | Interest Rate Swaps | Cash Flow Hedging
Derivative
Derivative, Notional Amount	120.0	127.3
Designated as Hedging Instrument | Interest Rate Swaps | Fair Value Hedging
Derivative
Derivative, Notional Amount	822.6	768.6
Designated as Hedging Instrument | Forward Benchmark Interest Rate Lock | Cash Flow Hedging
Derivative
Derivative, Notional Amount	2,526.0	2,603.0
Designated as Hedging Instrument | Derivative Instrument by Balance Sheet Location: us-gaap:OtherLiabilities
Derivative
Liability Derivatives Fair Value	246.7	253.6
Designated as Hedging Instrument | Derivative Instrument by Balance Sheet Location: us-gaap:OtherLiabilities | Cash Flow Hedging
Derivative
Liability Derivatives Fair Value	219.8	224.6
Designated as Hedging Instrument | Derivative Instrument by Balance Sheet Location: us-gaap:OtherLiabilities | Interest Rate Swaps | Cash Flow Hedging
Derivative
Liability Derivatives Fair Value	1.1	1.7
Designated as Hedging Instrument | Derivative Instrument by Balance Sheet Location: us-gaap:OtherLiabilities | Interest Rate Swaps | Fair Value Hedging
Derivative
Liability Derivatives Fair Value	26.9	29.0
Designated as Hedging Instrument | Derivative Instrument by Balance Sheet Location: us-gaap:OtherLiabilities | Forward Benchmark Interest Rate Lock | Cash Flow Hedging
Derivative
Liability Derivatives Fair Value	218.7	222.9
Designated as Hedging Instrument | Derivative Instrument by Balance Sheet Location: us-gaap:OtherLongTermInvestments
Derivative
Asset Derivatives Fair Value	59.7	51.6
Designated as Hedging Instrument | Derivative Instrument by Balance Sheet Location: us-gaap:OtherLongTermInvestments | Cash Flow Hedging
Derivative
Asset Derivatives Fair Value	15.2	16.6
Designated as Hedging Instrument | Derivative Instrument by Balance Sheet Location: us-gaap:OtherLongTermInvestments | Interest Rate Swaps | Cash Flow Hedging
Derivative
Asset Derivatives Fair Value	14.3	14.3
Designated as Hedging Instrument | Derivative Instrument by Balance Sheet Location: us-gaap:OtherLongTermInvestments | Interest Rate Swaps | Fair Value Hedging
Derivative
Asset Derivatives Fair Value	44.5	35.0
Designated as Hedging Instrument | Derivative Instrument by Balance Sheet Location: us-gaap:OtherLongTermInvestments | Forward Benchmark Interest Rate Lock | Cash Flow Hedging
Derivative
Asset Derivatives Fair Value	0.9	2.3
Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	300.8	319.3
Not Designated as Hedging Instrument | Interest Rate Swaps
Derivative
Derivative, Notional Amount	102.3	115.6
Not Designated as Hedging Instrument | Total Return Swap
Derivative
Derivative, Notional Amount	151.4	154.8
Not Designated as Hedging Instrument | Embedded Derivative in Modified Coinsurance Arrangement
Derivative
Derivative, Notional Amount	0.0	0.0
Not Designated as Hedging Instrument | Foreign Exchange Forward
Derivative
Derivative, Notional Amount	47.1	48.9
Not Designated as Hedging Instrument | Derivative Instrument by Balance Sheet Location: us-gaap:OtherLiabilities
Derivative
Liability Derivatives Fair Value	14.3	16.1
Not Designated as Hedging Instrument | Derivative Instrument by Balance Sheet Location: us-gaap:OtherLiabilities | Interest Rate Swaps
Derivative
Liability Derivatives Fair Value	13.6	15.1
Not Designated as Hedging Instrument | Derivative Instrument by Balance Sheet Location: us-gaap:OtherLiabilities | Total Return Swap
Derivative
Liability Derivatives Fair Value	0.0	0.0
Not Designated as Hedging Instrument | Derivative Instrument by Balance Sheet Location: us-gaap:OtherLiabilities | Embedded Derivative in Modified Coinsurance Arrangement
Derivative
Liability Derivatives Fair Value	0.0	0.0
Not Designated as Hedging Instrument | Derivative Instrument by Balance Sheet Location: us-gaap:OtherLiabilities | Foreign Exchange Forward
Derivative
Liability Derivatives Fair Value	0.7	1.0
Not Designated as Hedging Instrument | Derivative Instrument by Balance Sheet Location: us-gaap:OtherLongTermInvestments
Derivative
Asset Derivatives Fair Value	15.8	18.1
Not Designated as Hedging Instrument | Derivative Instrument by Balance Sheet Location: us-gaap:OtherLongTermInvestments | Interest Rate Swaps
Derivative
Asset Derivatives Fair Value	0.2	0.2
Not Designated as Hedging Instrument | Derivative Instrument by Balance Sheet Location: us-gaap:OtherLongTermInvestments | Total Return Swap
Derivative
Asset Derivatives Fair Value	0.0	0.0
Not Designated as Hedging Instrument | Derivative Instrument by Balance Sheet Location: us-gaap:OtherLongTermInvestments | Embedded Derivative in Modified Coinsurance Arrangement
Derivative
Asset Derivatives Fair Value	14.8	17.8
Not Designated as Hedging Instrument | Derivative Instrument by Balance Sheet Location: us-gaap:OtherLongTermInvestments | Foreign Exchange Forward
Derivative
Asset Derivatives Fair Value	$ 0.8	$ 0.1