Financial Instruments Carried at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Contracts	$ 34.4	$ 12.6
Money Market Funds	228.6	188.3
Fixed maturity and open ended mutual funds	32.2	31.9
Total, Assets	295.2	232.8
Derivatives, Liabilities	13.5	12.4
Total, Liabilities	13.5	12.4
Fair Value Inputs Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Money Market Funds	228.6	188.3
Fixed maturity and open ended mutual funds	32.2	31.9
Total, Assets	260.8	220.2
Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Contracts	34.4	12.6
Total, Assets	34.4	12.6
Derivatives, Liabilities	13.5	12.4
Total, Liabilities	$ 13.5	$ 12.4