Financial Instruments Carried at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Contracts	$ 59.3	$ 7.6
Money market funds	247.0	189.0
Fixed maturity and open ended mutual funds	34.4	32.6
Total, Assets	340.7	229.2
Derivatives, Liabilities	2.8	5.4
Total, Liabilities	2.8	5.4
Fair Value Inputs Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Money market funds	247.0	189.0
Fixed maturity and open ended mutual funds	34.4	32.6
Total, Assets	281.4	221.6
Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Contracts	59.3	7.6
Total, Assets	59.3	7.6
Derivatives, Liabilities	2.8	5.4
Total, Liabilities	$ 2.8	$ 5.4