Financial Instruments Carried at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Contracts	$ 14.5	$ 13.0
Money market funds	24.8	42.2
Fixed maturity and open ended mutual funds	39.7	21.1
Total, Assets	79.0	76.3
Derivatives, Liabilities	16.3	5.5
Total, Liabilities	16.3	5.5
Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Money market funds	24.8	42.2
Fixed maturity and open ended mutual funds	39.7	21.1
Total, Assets	64.5	63.3
Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Contracts	14.5	13.0
Total, Assets	14.5	13.0
Derivatives, Liabilities	16.3	5.5
Total, Liabilities	$ 16.3	$ 5.5