Financial Instruments Carried at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative contracts	$ 13.0	$ 7.6
Money Market Funds at Carrying Value	42.2	189.0
Fixed maturity and open ended mutual funds	21.1	32.6
Total, assets	76.3	229.2
Derivatives, liabilities	5.5	5.4
Total, liabilities	5.5	5.4
Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Money Market Funds at Carrying Value	42.2	189.0
Fixed maturity and open ended mutual funds	21.1	32.6
Total, assets	63.3	221.6
Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative contracts	13.0	7.6
Total, assets	13.0	7.6
Derivatives, liabilities	5.5	5.4
Total, liabilities	$ 5.5	$ 5.4