Location and Fair Values of Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Derivative
Asset Derivatives Fair Value	$ 24.6	$ 30.9
Designated as Hedging Instrument | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	24.5	30.4
Designated as Hedging Instrument | Other Liabilities
Derivative
Liability Derivatives Fair Value	14.0	13.5
Not Designated as Hedging Instrument | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	0.1	0.5
Not Designated as Hedging Instrument | Other Liabilities
Derivative
Liability Derivatives Fair Value	47.3	55.6
Not Designated as Hedging Instrument | Foreign Exchange Contracts | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	0.0	0.0
Not Designated as Hedging Instrument | Foreign Exchange Contracts | Other Liabilities
Derivative
Liability Derivatives Fair Value	21.7	24.5
Not Designated as Hedging Instrument | Credit Risk Contract | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	0.1	0.5
Not Designated as Hedging Instrument | Credit Risk Contract | Other Liabilities
Derivative
Liability Derivatives Fair Value	0.0	0.0
Not Designated as Hedging Instrument | Embedded Derivative in Modified Coinsurance Arrangement | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	0.0	0.0
Not Designated as Hedging Instrument | Embedded Derivative in Modified Coinsurance Arrangement | Other Liabilities
Derivative
Liability Derivatives Fair Value	25.6	31.1
Cash Flow Hedging | Designated as Hedging Instrument | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	19.4
Cash Flow Hedging | Designated as Hedging Instrument | Other Liabilities
Derivative
Liability Derivatives Fair Value	9.9
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	19.4	25.0
Cash Flow Hedging | Designated as Hedging Instrument | Foreign Exchange Contracts | Other Liabilities
Derivative
Liability Derivatives Fair Value	9.8	7.9
Fair Value Hedging | Designated as Hedging Instrument | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	5.1	5.4
Fair Value Hedging | Designated as Hedging Instrument | Other Liabilities
Derivative
Liability Derivatives Fair Value	4.1	5.6
Fair Value Hedging | Designated as Hedging Instrument | Foreign Exchange Contracts | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	5.1	5.4
Liability Derivatives Fair Value		0.4
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 | 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	$ 3.7	$ 5.2