Location and Fair Values of Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Derivative
Derivative, Notional Amount	$ 3,665.5	$ 2,983.7
Other Long-term Investments
Derivative
Asset Derivatives Fair Value	110.1	99.9
Other Liabilities
Derivative
Liability Derivatives Fair Value	130.2	117.7
Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	3,358.7	2,697.0
Designated as Hedging Instrument \| Cash Flow Hedging
Derivative
Derivative, Notional Amount	2,665.5	2,054.5
Designated as Hedging Instrument \| Forwards \| Cash Flow Hedging
Derivative
Derivative, Notional Amount	2,523.0	1,905.0
Designated as Hedging Instrument \| Interest Rate Swaps \| Cash Flow Hedging
Derivative
Derivative, Notional Amount	142.5	149.5
Designated as Hedging Instrument \| Interest Rate Swaps \| Fair Value Hedging
Derivative
Derivative, Notional Amount	693.2	642.5
Designated as Hedging Instrument \| Other Long-term Investments
Derivative
Asset Derivatives Fair Value	103.4	96.9
Designated as Hedging Instrument \| Other Long-term Investments \| Cash Flow Hedging
Derivative
Asset Derivatives Fair Value	62.3	58.7
Designated as Hedging Instrument \| Other Long-term Investments \| Forwards \| Cash Flow Hedging
Derivative
Asset Derivatives Fair Value	47.4	44.5
Designated as Hedging Instrument \| Other Long-term Investments \| Interest Rate Swaps \| Cash Flow Hedging
Derivative
Asset Derivatives Fair Value	14.9	14.2
Designated as Hedging Instrument \| Other Long-term Investments \| Interest Rate Swaps \| Fair Value Hedging
Derivative
Asset Derivatives Fair Value	41.1	38.2
Designated as Hedging Instrument \| Other Liabilities
Derivative
Liability Derivatives Fair Value	113.4	99.0
Designated as Hedging Instrument \| Other Liabilities \| Cash Flow Hedging
Derivative
Liability Derivatives Fair Value	97.7	82.3
Designated as Hedging Instrument \| Other Liabilities \| Forwards \| Cash Flow Hedging
Derivative
Liability Derivatives Fair Value	94.7	77.8
Designated as Hedging Instrument \| Other Liabilities \| Interest Rate Swaps \| Cash Flow Hedging
Derivative
Liability Derivatives Fair Value	3.0	4.5
Designated as Hedging Instrument \| Other Liabilities \| Interest Rate Swaps \| Fair Value Hedging
Derivative
Liability Derivatives Fair Value	15.7	16.7
Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	306.8	286.7
Not Designated as Hedging Instrument \| Forwards
Derivative
Derivative, Notional Amount	54.6	52.5
Not Designated as Hedging Instrument \| Interest Rate Swaps
Derivative
Derivative, Notional Amount	125.9	132.0
Not Designated as Hedging Instrument \| Total Return Swap
Derivative
Derivative, Notional Amount	126.3	102.2
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	6.7	3.0
Not Designated as Hedging Instrument \| Other Long-term Investments \| Forwards
Derivative
Asset Derivatives Fair Value	2.1	3.0
Not Designated as Hedging Instrument \| Other Long-term Investments \| Interest Rate Swaps
Derivative
Asset Derivatives Fair Value	0.0	0.0
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	4.6	0.0
Not Designated as Hedging Instrument \| Other Liabilities
Derivative
Liability Derivatives Fair Value	16.8	18.7
Not Designated as Hedging Instrument \| Other Liabilities \| Forwards
Derivative
Liability Derivatives Fair Value	0.5	0.2
Not Designated as Hedging Instrument \| Other Liabilities \| Interest Rate Swaps
Derivative
Liability Derivatives Fair Value	16.3	17.0
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	$ 1.5

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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