Location and Fair Values of Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Derivative
Asset Derivatives Fair Value	$ 25.2	$ 19.5
Designated as Hedging Instrument | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	25.2	19.5
Designated as Hedging Instrument | Other Liabilities
Derivative
Liability Derivatives Fair Value	20.2	24.5
Not Designated as Hedging Instrument | Other Liabilities
Derivative
Liability Derivatives Fair Value	46.4	43.6
Not Designated as Hedging Instrument | Foreign Exchange Contracts | Other Liabilities
Derivative
Liability Derivatives Fair Value	26.5	27.5
Not Designated as Hedging Instrument | Credit Default Swaps | 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	19.9	15.9
Cash Flow Hedging | Designated as Hedging Instrument | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	22.8	19.5
Cash Flow Hedging | Designated as Hedging Instrument | Other Liabilities
Derivative
Liability Derivatives Fair Value	12.8	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	0.1
Cash Flow Hedging | Designated as Hedging Instrument | Foreign Exchange Contracts | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	22.8	19.5
Cash Flow Hedging | Designated as Hedging Instrument | Foreign Exchange Contracts | Other Liabilities
Derivative
Liability Derivatives Fair Value	12.7	19.4
Fair Value Hedging | Designated as Hedging Instrument | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	2.4	0.0
Fair Value Hedging | Designated as Hedging Instrument | Other Liabilities
Derivative
Liability Derivatives Fair Value	7.4	5.1
Fair Value Hedging | Designated as Hedging Instrument | Foreign Exchange Contracts | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	2.4
Fair Value Hedging | Designated as Hedging Instrument | Foreign Exchange Contracts | Other Liabilities
Derivative
Liability Derivatives Fair Value	0.1
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	$ 7.3	$ 5.1