Location and Fair Values of Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Derivative
Derivative, Notional Amount	$ 2,324.2	$ 1,753.9
Asset Derivatives Fair Value	74.8	89.1
Other Long-term Investments
Derivative
Asset Derivatives Fair Value	75.7	89.1
Other Liabilities
Derivative
Liability Derivatives Fair Value	212.7	87.9
Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	2,050.0	1,490.7
Designated as Hedging Instrument \| Cash Flow Hedging
Derivative
Derivative, Notional Amount	1,446.4	932.9
Designated as Hedging Instrument \| Forwards \| Cash Flow Hedging
Derivative
Derivative, Notional Amount	1,290.0	764.0
Designated as Hedging Instrument \| Interest Rate Swaps \| Cash Flow Hedging
Derivative
Derivative, Notional Amount	156.4	168.9
Designated as Hedging Instrument \| Interest Rate Swaps \| Fair Value Hedging
Derivative
Derivative, Notional Amount	603.6	557.8
Designated as Hedging Instrument \| Other Long-term Investments
Derivative
Asset Derivatives Fair Value	69.8	85.1
Designated as Hedging Instrument \| Other Long-term Investments \| Cash Flow Hedging
Derivative
Asset Derivatives Fair Value	16.7	19.1
Designated as Hedging Instrument \| Other Long-term Investments \| Forwards \| Cash Flow Hedging
Derivative
Asset Derivatives Fair Value	0.0	1.6
Designated as Hedging Instrument \| Other Long-term Investments \| Interest Rate Swaps \| Cash Flow Hedging
Derivative
Asset Derivatives Fair Value	16.7	17.5
Designated as Hedging Instrument \| Other Long-term Investments \| Interest Rate Swaps \| Fair Value Hedging
Derivative
Asset Derivatives Fair Value	53.1	66.0
Designated as Hedging Instrument \| Other Liabilities
Derivative
Liability Derivatives Fair Value	196.3	56.9
Designated as Hedging Instrument \| Other Liabilities \| Cash Flow Hedging
Derivative
Liability Derivatives Fair Value	186.6	51.5
Designated as Hedging Instrument \| Other Liabilities \| Forwards \| Cash Flow Hedging
Derivative
Liability Derivatives Fair Value	184.1	48.4
Designated as Hedging Instrument \| Other Liabilities \| Interest Rate Swaps \| Cash Flow Hedging
Derivative
Liability Derivatives Fair Value	2.5	3.1
Designated as Hedging Instrument \| Other Liabilities \| Interest Rate Swaps \| Fair Value Hedging
Derivative
Liability Derivatives Fair Value	9.7	5.4
Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	274.2	263.2
Not Designated as Hedging Instrument \| Forwards
Derivative
Derivative, Notional Amount	53.2	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	89.0	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	5.9	4.0
Not Designated as Hedging Instrument \| Other Long-term Investments \| Forwards
Derivative
Asset Derivatives Fair Value	5.0	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	0.9	0.0
Not Designated as Hedging Instrument \| Other Liabilities
Derivative
Liability Derivatives Fair Value	16.4	31.0
Not Designated as Hedging Instrument \| Other Liabilities \| Forwards
Derivative
Liability Derivatives Fair Value	0.0	0.1
Not Designated as Hedging Instrument \| Other Liabilities \| Interest Rate Swaps
Derivative
Liability Derivatives Fair Value	16.4	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	$ 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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