Cash Equivalent and Investments (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014
Schedule Of Available For Sale Securities [Line Items]
Fair value	$ 32.9	$ 31.9
Cash and cash equivalents	1,746.1	1,757.4	$ 1,471.1	$ 1,219.5
Short-term investments	311.8	474.8
Money Market [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost	38.9	188.3
Fair value	38.9	188.3
Cash and cash equivalents	38.9	188.3
Certificates Of Deposit [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost	798.0	1,307.3
Fair value	798.0	1,307.3
Cash and cash equivalents	458.1	809.4
Short-term investments	311.8	474.8
Other assets	28.1	23.1
Fixed Maturity and Mutual Funds [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost	27.9	28.7
Gross unrealized gain	5.0	3.2
Fair value	32.9	31.9
Other assets	$ 32.9	$ 31.9