Location and Fair Values of Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Sep. 30, 2026	Dec. 31, 2025
Derivative
Derivative, Notional Amount	$ 5,157.0	$ 3,818.2
Asset Derivatives Fair Value	69.0	69.7
Liability Derivatives Fair Value	421.5	269.7
Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	3,268.2	3,498.9
Asset Derivatives Fair Value	44.6	51.6
Liability Derivatives Fair Value	344.6	253.6
Designated as Hedging Instrument | Cash Flow Hedging
Derivative
Derivative, Notional Amount	2,420.2	2,730.3
Asset Derivatives Fair Value	11.2	16.6
Liability Derivatives Fair Value	318.1	224.6
Designated as Hedging Instrument | Forward Benchmark Interest Rate Lock | Cash Flow Hedging
Derivative
Derivative, Notional Amount	2,311.0	2,603.0
Asset Derivatives Fair Value	0.0	2.3
Liability Derivatives Fair Value	317.2	222.9
Designated as Hedging Instrument | Receive Fixed/Pay Fixed | Cash Flow Hedging
Derivative
Derivative, Notional Amount	109.2	127.3
Asset Derivatives Fair Value	11.2	14.3
Liability Derivatives Fair Value	0.9	1.7
Designated as Hedging Instrument | Receive Fixed/Pay Fixed | Fair Value Hedging
Derivative
Derivative, Notional Amount	848.0	768.6
Asset Derivatives Fair Value	33.4	35.0
Liability Derivatives Fair Value	26.5	29.0
Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	1,888.8	319.3
Asset Derivatives Fair Value	24.4	18.1
Liability Derivatives Fair Value	76.9	16.1
Not Designated as Hedging Instrument | Forward Benchmark Interest Rate Lock
Derivative
Derivative, Notional Amount	832.0
Asset Derivatives Fair Value	0.0
Liability Derivatives Fair Value	45.2
Not Designated as Hedging Instrument | Interest Rate Swaps
Derivative
Derivative, Notional Amount	205.8
Asset Derivatives Fair Value	4.7
Liability Derivatives Fair Value	0.0
Not Designated as Hedging Instrument | Foreign Exchange Forward
Derivative
Derivative, Notional Amount	45.8	48.9
Asset Derivatives Fair Value	1.0	0.1
Liability Derivatives Fair Value	0.4	1.0
Not Designated as Hedging Instrument | Total Return Swap
Derivative
Derivative, Notional Amount	722.3	154.8
Asset Derivatives Fair Value	0.0	0.0
Liability Derivatives Fair Value	20.6	0.0
Not Designated as Hedging Instrument | Embedded Derivative in Modified Coinsurance Arrangement
Derivative
Derivative, Notional Amount	0.0	0.0
Asset Derivatives Fair Value	18.7	17.8
Liability Derivatives Fair Value	0.0	0.0
Not Designated as Hedging Instrument | Receive Fixed/Pay Fixed
Derivative
Derivative, Notional Amount	82.9	115.6
Asset Derivatives Fair Value	0.0	0.2
Liability Derivatives Fair Value	$ 10.7	$ 15.1