Cash Equivalent and Investments (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
Schedule Of Available For Sale Securities [Line Items]
Fair value	$ 32.2	$ 31.9
Cash and cash equivalents	1,576.1	1,757.4	$ 1,509.9	$ 1,219.5
Short-term investments	490.6	474.8
Money Market [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost	$ 228.6	188.3
Gross unrealized gain
Fair value	$ 228.6	188.3
Cash and cash equivalents	$ 228.6	188.3
Short-term investments
Other assets
Certificates Of Deposit [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost	$ 1,097.4	1,307.3
Gross unrealized gain
Fair value	$ 1,097.4	1,307.3
Cash and cash equivalents	583.9	809.4
Short-term investments	490.6	474.8
Other assets	22.9	23.1
Fixed Maturity and Mutual Funds [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost	28.5	28.7
Gross unrealized gain	3.7	3.2
Fair value	$ 32.2	31.9
Cash and cash equivalents
Short-term investments
Other assets	$ 32.2	$ 31.9