Financial Instruments Carried at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Contracts	$ 5.8	$ 13.0
Money market funds	35.6	42.2
Fixed maturity and open ended mutual funds	42.4	21.1
Total, Assets	83.8	76.3
Derivatives, Liabilities	47.8	5.5
Total, Liabilities	47.8	5.5
Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Money market funds	35.6	42.2
Fixed maturity and open ended mutual funds	42.4	21.1
Total, Assets	78.0	63.3
Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Contracts	5.8	13.0
Total, Assets	5.8	13.0
Derivatives, Liabilities	47.8	5.5
Total, Liabilities	$ 47.8	$ 5.5