Location and Fair Values of Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Derivative
Asset Derivatives Fair Value	$ 27.5	$ 30.9
Designated as Hedging Instrument | Miscellaneous Long-term Investments
Derivative
Asset Derivatives Fair Value	27.0	30.4
Designated as Hedging Instrument | Miscellaneous Long-term Investments | Fair Value Hedging
Derivative
Asset Derivatives Fair Value	7.6	5.4
Designated as Hedging Instrument | Miscellaneous Long-term Investments | Foreign Exchange Contracts | Cash Flow Hedging
Derivative
Asset Derivatives Fair Value	19.4	25.0
Designated as Hedging Instrument | Miscellaneous Long-term Investments | Foreign Exchange Contracts | Fair Value Hedging
Derivative
Asset Derivatives Fair Value	7.6	5.4
Designated as Hedging Instrument | Miscellaneous 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	12.2	13.5
Designated as Hedging Instrument | Other Liabilities | Fair Value Hedging
Derivative
Liability Derivatives Fair Value	5.6	5.6
Designated as Hedging Instrument | Other Liabilities | Foreign Exchange Contracts | Cash Flow Hedging
Derivative
Liability Derivatives Fair Value	6.6	7.9
Designated as Hedging Instrument | Other Liabilities | Foreign Exchange Contracts | Fair Value Hedging
Derivative
Liability Derivatives Fair Value	5.0	0.4
Designated as Hedging Instrument | Other Liabilities | Interest Rate Swaps | Fair Value Hedging
Derivative
Liability Derivatives Fair Value	0.6	5.2
Not Designated as Hedging Instrument | Miscellaneous Long-term Investments
Derivative
Asset Derivatives Fair Value	0.5	0.5
Not Designated as Hedging Instrument | Miscellaneous Long-term Investments | Foreign Exchange Contracts
Derivative
Asset Derivatives Fair Value	0.0	0.0
Not Designated as Hedging Instrument | Miscellaneous Long-term Investments | Credit Default Swaps
Derivative
Asset Derivatives Fair Value	0.5	0.5
Not Designated as Hedging Instrument | Miscellaneous 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	45.2	55.6
Not Designated as Hedging Instrument | Other Liabilities | Foreign Exchange Contracts
Derivative
Liability Derivatives Fair Value	22.4	24.5
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	$ 22.8	$ 31.1