Location and Fair Values of Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Derivative
Asset Derivatives Fair Value	$ 19.5	$ 32.7
Designated as Hedging Instrument | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	19.5	32.7
Designated as Hedging Instrument | Other Liability
Derivative
Liability Derivatives Fair Value	24.5	20.3
Designated as Hedging Instrument | Interest Rate Swaps | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	0.0	0.0
Designated as Hedging Instrument | Interest Rate Swaps | Other Liability
Derivative
Liability Derivatives Fair Value	5.1	6.9
Designated as Hedging Instrument | Foreign Exchange Contracts | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	19.5	32.7
Designated as Hedging Instrument | Foreign Exchange Contracts | Other Liability
Derivative
Liability Derivatives Fair Value	19.4	13.4
Not Designated as Hedging Instrument | Other Liability
Derivative
Liability Derivatives Fair Value	43.6	79.2
Not Designated as Hedging Instrument | Interest Rate Swaps | Other Liability
Derivative
Liability Derivatives Fair Value		0.7
Not Designated as Hedging Instrument | Foreign Exchange Contracts | Other Liability
Derivative
Liability Derivatives Fair Value	27.5	31.4
Not Designated as Hedging Instrument | Credit Risk Contract | Other Liability
Derivative
Liability Derivatives Fair Value	0.2	0.4
Not Designated as Hedging Instrument | Embedded Derivative in Modified Coinsurance Arrangement | Other Liability
Derivative
Liability Derivatives Fair Value	$ 15.9	$ 46.7