Cash Equivalent and Investments (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Money Market [Member]
Schedule of Available-for-sale Securities [Line Item]
Cost	$ 42.2	$ 189.0
Fair Value	42.2	189.0
Cash and Cash Equivalents	42.2	189.0
Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Item]
Cost	351.4	1,190.5
Fair Value	351.4	1,190.5
Cash and Cash Equivalents	238.6	1,017.0
Short Term Investments	111.8	173.4
Other Assets	1.0	0.1
Fixed Maturity and Mutual Funds [Member]
Schedule of Available-for-sale Securities [Line Item]
Cost	16.8	27.0
Gross Unrealized Gain	4.3	5.6
Fair Value	21.1	32.6
Other Assets	$ 21.1	$ 32.6