Location and Fair Values of Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Derivative
Asset Derivatives Fair Value	$ 30.9	$ 19.5
Designated as Hedging Instrument | Miscellaneous Long-term Investments
Derivative
Asset Derivatives Fair Value	30.4	19.5
Designated as Hedging Instrument | Other Liabilities
Derivative
Liability Derivatives Fair Value	13.5	24.5
Not Designated as Hedging Instrument | Miscellaneous Long-term Investments
Derivative
Asset Derivatives Fair Value	0.5
Not Designated as Hedging Instrument | Other Liabilities
Derivative
Liability Derivatives Fair Value	55.6	43.6
Not Designated as Hedging Instrument | Foreign Exchange Contracts | Miscellaneous Long-term Investments
Derivative
Asset Derivatives Fair Value	0.0
Not Designated as Hedging Instrument | Foreign Exchange Contracts | Other Liabilities
Derivative
Liability Derivatives Fair Value	24.5	27.5
Not Designated as Hedging Instrument | Credit Default Swaps | Miscellaneous Long-term Investments
Derivative
Asset Derivatives Fair Value	0.5
Not Designated as Hedging Instrument | Credit Default Swaps | Other Liabilities
Derivative
Liability Derivatives Fair Value	0.0	0.2
Not Designated as Hedging Instrument | Embedded Derivative in Modified Coinsurance Arrangement | Miscellaneous Long-term Investments
Derivative
Asset Derivatives Fair Value	0.0
Not Designated as Hedging Instrument | Embedded Derivative in Modified Coinsurance Arrangement | Other Liabilities
Derivative
Liability Derivatives Fair Value	31.1	15.9
Cash Flow Hedging | Designated as Hedging Instrument | Foreign Exchange Contracts | Miscellaneous Long-term Investments
Derivative
Asset Derivatives Fair Value	25.0	19.5
Cash Flow Hedging | Designated as Hedging Instrument | Foreign Exchange Contracts | Other Liabilities
Derivative
Liability Derivatives Fair Value	7.9	19.4
Fair Value Hedging | Designated as Hedging Instrument | Miscellaneous Long-term Investments
Derivative
Asset Derivatives Fair Value	5.4
Fair Value Hedging | Designated as Hedging Instrument | Other Liabilities
Derivative
Liability Derivatives Fair Value	5.6
Fair Value Hedging | Designated as Hedging Instrument | Foreign Exchange Contracts | Miscellaneous Long-term Investments
Derivative
Asset Derivatives Fair Value	5.4
Fair Value Hedging | Designated as Hedging Instrument | Foreign Exchange Contracts | Other Liabilities
Derivative
Liability Derivatives Fair Value	0.4
Fair Value Hedging | Designated as Hedging Instrument | Interest Rate Swaps | Miscellaneous Long-term Investments
Derivative
Asset Derivatives Fair Value		0.0
Fair Value Hedging | Designated as Hedging Instrument | Interest Rate Swaps | Other Liabilities
Derivative
Liability Derivatives Fair Value	$ 5.2	$ 5.1