Financial Instruments Carried at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Contracts	$ 22.9	$ 7.6
Money market funds	18.5	189.0
Fixed maturity and open ended mutual funds	27.0	32.6
Total, Assets	68.4	229.2
Derivatives, Liabilities	5.1	5.4
Total, Liabilities	5.1	5.4
Fair Value Inputs Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Money market funds	18.5	189.0
Fixed maturity and open ended mutual funds	27.0	32.6
Total, Assets	45.5	221.6
Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Contracts	22.9	7.6
Total, Assets	22.9	7.6
Derivatives, Liabilities	5.1	5.4
Total, Liabilities	$ 5.1	$ 5.4