Cash Equivalent and Investments (Detail) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015
Schedule Of Available For Sale Securities [Line Items]
Fair value	$ 34.4	$ 32.6
Cash and cash equivalents	3,280.9	2,051.5	$ 1,674.3	$ 1,757.4
Short-term investments	86.3	173.4
Money Market [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost	247.0	189.0
Fair value	247.0	189.0
Cash and cash equivalents	247.0	189.0
Certificates Of Deposit [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost	2,150.2	1,190.5
Fair value	2,150.2	1,190.5
Cash and cash equivalents	2,060.9	1,017.0
Short-term investments	86.3	173.4
Other assets	3.0	0.1
Fixed Maturity and Mutual Funds [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost	27.6	27.0
Gross unrealized gain	6.8	5.6
Fair value	34.4	32.6
Other assets	$ 34.4	$ 32.6