Location and Fair Values of Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Derivative
Derivative, Notional Amount	$ 3,851.1	$ 3,751.4
Other Long-term Investments
Derivative
Asset Derivatives Fair Value	70.5	90.9
Other Liabilities
Derivative
Liability Derivatives Fair Value	259.6	255.7
Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	3,532.3	3,445.5
Designated as Hedging Instrument \| Cash Flow Hedging
Derivative
Derivative, Notional Amount	2,763.6	2,709.1
Designated as Hedging Instrument \| Forward Benchmark Interest Rate Lock \| Cash Flow Hedging
Derivative
Derivative, Notional Amount	2,634.0	2,570.0
Designated as Hedging Instrument \| Interest Rate Swaps \| Cash Flow Hedging
Derivative
Derivative, Notional Amount	129.6	139.1
Designated as Hedging Instrument \| Interest Rate Swaps \| Fair Value Hedging
Derivative
Derivative, Notional Amount	768.7	736.4
Designated as Hedging Instrument \| Other Long-term Investments
Derivative
Asset Derivatives Fair Value	56.3	75.7
Designated as Hedging Instrument \| Other Long-term Investments \| Cash Flow Hedging
Derivative
Asset Derivatives Fair Value	20.8	21.0
Designated as Hedging Instrument \| Other Long-term Investments \| Forward Benchmark Interest Rate Lock \| Cash Flow Hedging
Derivative
Asset Derivatives Fair Value	5.0	3.4
Designated as Hedging Instrument \| Other Long-term Investments \| Interest Rate Swaps \| Cash Flow Hedging
Derivative
Asset Derivatives Fair Value	15.8	17.6
Designated as Hedging Instrument \| Other Long-term Investments \| Interest Rate Swaps \| Fair Value Hedging
Derivative
Asset Derivatives Fair Value	35.5	54.7
Designated as Hedging Instrument \| Other Liabilities
Derivative
Liability Derivatives Fair Value	242.8	239.3
Designated as Hedging Instrument \| Other Liabilities \| Cash Flow Hedging
Derivative
Liability Derivatives Fair Value	208.5	224.3
Designated as Hedging Instrument \| Other Liabilities \| Forward Benchmark Interest Rate Lock \| Cash Flow Hedging
Derivative
Liability Derivatives Fair Value	207.1	223.2
Designated as Hedging Instrument \| Other Liabilities \| Interest Rate Swaps \| Cash Flow Hedging
Derivative
Liability Derivatives Fair Value	1.4	1.1
Designated as Hedging Instrument \| Other Liabilities \| Interest Rate Swaps \| Fair Value Hedging
Derivative
Liability Derivatives Fair Value	34.3	15.0
Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	318.8	305.9
Not Designated as Hedging Instrument \| Interest Rate Swaps
Derivative
Derivative, Notional Amount	119.7	125.9
Not Designated as Hedging Instrument \| Foreign Exchange Forward
Derivative
Derivative, Notional Amount	49.1	51.1
Not Designated as Hedging Instrument \| Total Return Swap
Derivative
Derivative, Notional Amount	150.0	128.9
Not Designated as Hedging Instrument \| Embedded Derivative in Modified Coinsurance Arrangement
Derivative
Derivative, Notional Amount	0.0	0.0
Not Designated as Hedging Instrument \| Other Long-term Investments
Derivative
Asset Derivatives Fair Value	14.2	15.2
Not Designated as Hedging Instrument \| Other Long-term Investments \| Interest Rate Swaps
Derivative
Asset Derivatives Fair Value	0.3	0.6
Not Designated as Hedging Instrument \| Other Long-term Investments \| Foreign Exchange Forward
Derivative
Asset Derivatives Fair Value	0.0	3.1
Not Designated as Hedging Instrument \| Other Long-term Investments \| Total Return Swap
Derivative
Asset Derivatives Fair Value	0.0	0.0
Not Designated as Hedging Instrument \| Other Long-term Investments \| Embedded Derivative in Modified Coinsurance Arrangement
Derivative
Asset Derivatives Fair Value	13.9	11.5
Not Designated as Hedging Instrument \| Other Liabilities
Derivative
Liability Derivatives Fair Value	16.8	16.4
Not Designated as Hedging Instrument \| Other Liabilities \| Interest Rate Swaps
Derivative
Liability Derivatives Fair Value	15.7	16.4
Not Designated as Hedging Instrument \| Other Liabilities \| Foreign Exchange Forward
Derivative
Liability Derivatives Fair Value	1.1	0.0
Not Designated as Hedging Instrument \| Other Liabilities \| Total Return Swap
Derivative
Liability Derivatives Fair Value	0.0	0.0
Not Designated as Hedging Instrument \| Other Liabilities \| Embedded Derivative in Modified Coinsurance Arrangement
Derivative
Liability Derivatives Fair Value	$ 0.0	$ 0.0

Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

Nominal or face amount used to calculate payment on derivative.

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