Cash Equivalent and Investments (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Cash and Cash Equivalents [Line Items]
Fair value	$ 27.0	$ 32.6
Cash and cash equivalents	962.8	2,051.5	$ 1,746.1	$ 1,757.4
Short-term investments	108.3	173.4
Money Market [Member]
Cash and Cash Equivalents [Line Items]
Cost	18.5	189.0
Fair value	18.5	189.0
Cash and cash equivalents	18.5	189.0
Certificates Of Deposit [Member]
Cash and Cash Equivalents [Line Items]
Cost	253.2	1,190.5
Fair value	253.2	1,190.5
Cash and cash equivalents	142.6	1,017.0
Short-term investments	108.3	173.4
Other assets	2.3	0.1
Fixed Maturity and Mutual Funds [Member]
Cash and Cash Equivalents [Line Items]
Cost	21.8	27.0
Gross unrealized gain	5.2	5.6
Fair value	27.0	32.6
Other assets	$ 27.0	$ 32.6