Location and Fair Values of Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Derivative
Asset Derivatives Fair Value	$ 20.9	$ 19.5
Designated as Hedging Instrument | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	20.4	19.5
Designated as Hedging Instrument | Other Liabilities
Derivative
Liability Derivatives Fair Value	22.7	24.5
Not Designated as Hedging Instrument | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	0.5
Not Designated as Hedging Instrument | Other Liabilities
Derivative
Liability Derivatives Fair Value	37.6	43.6
Not Designated as Hedging Instrument | Foreign Exchange Contracts | Other 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	23.8	27.5
Not Designated as Hedging Instrument | Credit Default Swaps | Other 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 | Other 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	13.8	15.9
Cash Flow Hedging | Designated as Hedging Instrument | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	18.3	19.5
Cash Flow Hedging | Designated as Hedging Instrument | Other Liabilities
Derivative
Liability Derivatives Fair Value	14.7	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	0.0
Cash Flow Hedging | Designated as Hedging Instrument | Foreign Exchange Contracts | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	18.3	19.5
Cash Flow Hedging | Designated as Hedging Instrument | Foreign Exchange Contracts | Other Liabilities
Derivative
Liability Derivatives Fair Value	14.7	19.4
Fair Value Hedging | Designated as Hedging Instrument | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	2.1	0.0
Fair Value Hedging | Designated as Hedging Instrument | Other Liabilities
Derivative
Liability Derivatives Fair Value	8.0	5.1
Fair Value Hedging | Designated as Hedging Instrument | Foreign Exchange Contracts | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	2.1
Fair Value Hedging | Designated as Hedging Instrument | Foreign Exchange Contracts | Other Liabilities
Derivative
Liability Derivatives Fair Value	0.7
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