Location and Fair Values of Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Derivative
Derivative, Notional Amount	$ 2,983.7	$ 1,753.9
Other Long-term Investments
Derivative
Asset Derivatives Fair Value	99.9	89.1
Other Liabilities
Derivative
Liability Derivatives Fair Value	117.7	87.9
Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	2,697.0	1,490.7
Designated as Hedging Instrument \| Cash Flow Hedging
Derivative
Derivative, Notional Amount	2,054.5	932.9
Designated as Hedging Instrument \| Forwards \| Cash Flow Hedging
Derivative
Derivative, Notional Amount	1,905.0	764.0
Designated as Hedging Instrument \| Interest Rate Swaps \| Cash Flow Hedging
Derivative
Derivative, Notional Amount	149.5	168.9
Designated as Hedging Instrument \| Interest Rate Swaps \| Fair Value Hedging
Derivative
Derivative, Notional Amount	642.5	557.8
Designated as Hedging Instrument \| Other Long-term Investments
Derivative
Asset Derivatives Fair Value	96.9	85.1
Designated as Hedging Instrument \| Other Long-term Investments \| Cash Flow Hedging
Derivative
Asset Derivatives Fair Value	58.7	19.1
Designated as Hedging Instrument \| Other Long-term Investments \| Forwards \| Cash Flow Hedging
Derivative
Asset Derivatives Fair Value	44.5	1.6
Designated as Hedging Instrument \| Other Long-term Investments \| Interest Rate Swaps \| Cash Flow Hedging
Derivative
Asset Derivatives Fair Value	14.2	17.5
Designated as Hedging Instrument \| Other Long-term Investments \| Interest Rate Swaps \| Fair Value Hedging
Derivative
Asset Derivatives Fair Value	38.2	66.0
Designated as Hedging Instrument \| Other Liabilities
Derivative
Liability Derivatives Fair Value	99.0	56.9
Designated as Hedging Instrument \| Other Liabilities \| Cash Flow Hedging
Derivative
Liability Derivatives Fair Value	82.3	51.5
Designated as Hedging Instrument \| Other Liabilities \| Forwards \| Cash Flow Hedging
Derivative
Liability Derivatives Fair Value	77.8	48.4
Designated as Hedging Instrument \| Other Liabilities \| Interest Rate Swaps \| Cash Flow Hedging
Derivative
Liability Derivatives Fair Value	4.5	3.1
Designated as Hedging Instrument \| Other Liabilities \| Interest Rate Swaps \| Fair Value Hedging
Derivative
Liability Derivatives Fair Value	16.7	5.4
Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	286.7	263.2
Not Designated as Hedging Instrument \| Forwards
Derivative
Derivative, Notional Amount	52.5	54.3
Not Designated as Hedging Instrument \| Interest Rate Swaps
Derivative
Derivative, Notional Amount	132.0	132.0
Not Designated as Hedging Instrument \| Total Return Swap
Derivative
Derivative, Notional Amount	102.2	76.9
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	3.0	4.0
Not Designated as Hedging Instrument \| Other Long-term Investments \| Forwards
Derivative
Asset Derivatives Fair Value	3.0	4.0
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	0.0	0.0
Not Designated as Hedging Instrument \| Other Liabilities
Derivative
Liability Derivatives Fair Value	18.7	31.0
Not Designated as Hedging Instrument \| Other Liabilities \| Forwards
Derivative
Liability Derivatives Fair Value	0.2	0.1
Not Designated as Hedging Instrument \| Other Liabilities \| Interest Rate Swaps
Derivative
Liability Derivatives Fair Value	17.0	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	$ 1.5	$ 13.9

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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