Cash Equivalent and Investments (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Line Items]
Fair value	$ 39.7	$ 21.1
Cash and cash equivalents	1,277.3	1,071.5	$ 2,129.6	$ 2,051.5
Short-term investments	100.5	111.8
Money Market [Member]
Cash and Cash Equivalents [Line Items]
Cost	24.8	42.2
Fair value	24.8	42.2
Cash and cash equivalents	24.8	42.2
Certificates Of Deposit [Member]
Cash and Cash Equivalents [Line Items]
Cost	281.7	351.4
Fair value	281.7	351.4
Cash and cash equivalents	176.6	238.6
Short-term investments	100.5	111.8
Other assets	4.6	1.0
Fixed Maturity and Mutual Funds [Member]
Cash and Cash Equivalents [Line Items]
Cost	35.0	16.8
Gross unrealized gain	4.7	4.3
Fair value	39.7	21.1
Other assets	$ 39.7	$ 21.1