Location and Fair Values of Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Jun. 30, 2026	Dec. 31, 2025
Derivative
Derivative, Notional Amount	$ 3,718.3	$ 3,818.2
Asset Derivatives Fair Value	73.0	69.7
Liability Derivatives Fair Value	223.0	269.7
Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	3,411.7	3,498.9
Asset Derivatives Fair Value	57.3	51.6
Liability Derivatives Fair Value	210.7	253.6
Designated as Hedging Instrument | Cash Flow Hedging
Derivative
Derivative, Notional Amount	2,529.6	2,730.3
Asset Derivatives Fair Value	16.1	16.6
Liability Derivatives Fair Value	183.6	224.6
Designated as Hedging Instrument | Forward Benchmark Interest Rate Lock | Cash Flow Hedging
Derivative
Derivative, Notional Amount	2,417.0	2,603.0
Asset Derivatives Fair Value	3.3	2.3
Liability Derivatives Fair Value	182.7	222.9
Designated as Hedging Instrument | Interest Rate Swaps | Cash Flow Hedging
Derivative
Derivative, Notional Amount	112.6	127.3
Asset Derivatives Fair Value	12.8	14.3
Liability Derivatives Fair Value	0.9	1.7
Designated as Hedging Instrument | Interest Rate Swaps | Fair Value Hedging
Derivative
Derivative, Notional Amount	882.1	768.6
Asset Derivatives Fair Value	41.2	35.0
Liability Derivatives Fair Value	27.1	29.0
Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	306.6	319.3
Asset Derivatives Fair Value	15.7	18.1
Liability Derivatives Fair Value	12.3	16.1
Not Designated as Hedging Instrument | Interest Rate Swaps
Derivative
Derivative, Notional Amount	89.0	115.6
Asset Derivatives Fair Value	0.0	0.2
Liability Derivatives Fair Value	11.8	15.1
Not Designated as Hedging Instrument | Foreign Exchange Forward
Derivative
Derivative, Notional Amount	45.8	48.9
Asset Derivatives Fair Value	0.6	0.1
Liability Derivatives Fair Value	0.5	1.0
Not Designated as Hedging Instrument | Total Return Swap
Derivative
Derivative, Notional Amount	171.8	154.8
Asset Derivatives Fair Value	0.0	0.0
Liability Derivatives Fair Value	0.0	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	15.1	17.8
Liability Derivatives Fair Value	$ 0.0	$ 0.0