Location and Fair Values of Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Derivative
Asset Derivatives Fair Value	$ 21.5	$ 19.5
Designated as Hedging Instrument | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	21.5	19.5
Designated as Hedging Instrument | Other Liabilities
Derivative
Liability Derivatives Fair Value	24.6	24.5
Not Designated as Hedging Instrument | Other Liabilities
Derivative
Liability Derivatives Fair Value	44.4	43.6
Not Designated as Hedging Instrument | Foreign Exchange Contracts | Other Liabilities
Derivative
Liability Derivatives Fair Value	26.8	27.5
Not Designated as Hedging Instrument | Credit Risk Contract | Other Liabilities
Derivative
Liability Derivatives Fair Value		0.2
Not Designated as Hedging Instrument | Embedded Derivative in Modified Coinsurance Arrangement | Other Liabilities
Derivative
Liability Derivatives Fair Value	17.6	15.9
Cash Flow Hedging | Designated as Hedging Instrument | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	21.2	19.5
Cash Flow Hedging | Designated as Hedging Instrument | Other Liabilities
Derivative
Liability Derivatives Fair Value	17.6	19.4
Cash Flow Hedging | Designated as Hedging Instrument | Forwards | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	0.0
Cash Flow Hedging | Designated as Hedging Instrument | Forwards | Other Liabilities
Derivative
Liability Derivatives Fair Value	0.2
Cash Flow Hedging | Designated as Hedging Instrument | Foreign Exchange Contracts | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	21.2	19.5
Cash Flow Hedging | Designated as Hedging Instrument | Foreign Exchange Contracts | Other Liabilities
Derivative
Liability Derivatives Fair Value	17.4	19.4
Fair Value Hedging | Designated as Hedging Instrument | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	0.3	0.0
Fair Value Hedging | Designated as Hedging Instrument | Other Liabilities
Derivative
Liability Derivatives Fair Value	7.0	5.1
Fair Value Hedging | Designated as Hedging Instrument | Interest Rate Swaps | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	0.0	0.0
Fair Value Hedging | Designated as Hedging Instrument | Interest Rate Swaps | Other Liabilities
Derivative
Liability Derivatives Fair Value	7.0	$ 5.1
Fair Value Hedging | Designated as Hedging Instrument | Foreign Exchange Contracts | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	0.3
Fair Value Hedging | Designated as Hedging Instrument | Foreign Exchange Contracts | Other Liabilities
Derivative
Liability Derivatives Fair Value	$ 0.0