Cash Equivalent and Investments (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Schedule Of Available For Sale Securities [Line Items]
Fair value	$ 32.5	$ 32.6
Cash and cash equivalents	2,129.6	2,051.5	$ 1,576.1	$ 1,757.4
Short-term investments	132.8	173.4
Money Market [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost	240.8	189.0
Fair value	240.8	189.0
Cash and cash equivalents	240.8	189.0
Certificates Of Deposit [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost	1,111.5	1,190.5
Fair value	1,111.5	1,190.5
Cash and cash equivalents	977.9	1,017.0
Short-term investments	132.8	173.4
Other assets	0.8	0.1
Fixed Maturity and Mutual Funds [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost	26.5	27.0
Gross unrealized gain	6.0	5.6
Fair value	32.5	32.6
Other assets	$ 32.5	$ 32.6