Location and Fair Values of Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Derivative
Asset Derivatives Fair Value	$ 38.3	$ 27.5
Designated as Hedging Instrument | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	38.0	27.0
Designated as Hedging Instrument | Other Long-term Investments | Fair Value Hedging
Derivative
Asset Derivatives Fair Value		7.6
Designated as Hedging Instrument | Other Long-term Investments | Foreign Exchange Contracts | Cash Flow Hedging
Derivative
Asset Derivatives Fair Value	23.6	19.4
Designated as Hedging Instrument | Other Long-term Investments | Foreign Exchange Contracts | Fair Value Hedging
Derivative
Asset Derivatives Fair Value	14.4	7.6
Designated as Hedging Instrument | Other Long-term Investments | Interest Rate Swaps | Fair Value Hedging
Derivative
Asset Derivatives Fair Value		0.0
Designated as Hedging Instrument | Other Liabilities
Derivative
Liability Derivatives Fair Value	9.9	12.2
Designated as Hedging Instrument | Other Liabilities | Fair Value Hedging
Derivative
Liability Derivatives Fair Value		5.6
Designated as Hedging Instrument | Other Liabilities | Foreign Exchange Contracts | Cash Flow Hedging
Derivative
Liability Derivatives Fair Value	5.3	6.6
Designated as Hedging Instrument | Other Liabilities | Foreign Exchange Contracts | Fair Value Hedging
Derivative
Liability Derivatives Fair Value	4.6	5.0
Designated as Hedging Instrument | Other Liabilities | Interest Rate Swaps | Fair Value Hedging
Derivative
Liability Derivatives Fair Value		0.6
Not Designated as Hedging Instrument | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	0.3	0.5
Not Designated as Hedging Instrument | Other Long-term Investments | Forwards
Derivative
Asset Derivatives Fair Value	0.1
Not Designated as Hedging Instrument | Other Long-term Investments | Foreign Exchange Contracts
Derivative
Asset Derivatives Fair Value	0.0	0.0
Not Designated as Hedging Instrument | Other Long-term Investments | Credit Default Swaps
Derivative
Asset Derivatives Fair Value	0.2	0.5
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	77.8	45.2
Not Designated as Hedging Instrument | Other Liabilities | Forwards
Derivative
Liability Derivatives Fair Value	0.0
Not Designated as Hedging Instrument | Other Liabilities | Foreign Exchange Contracts
Derivative
Liability Derivatives Fair Value	24.1	22.4
Not Designated as Hedging Instrument | Other Liabilities | Credit Default Swaps
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	$ 53.7	$ 22.8