Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Statement of Income and Comprehensive Income [Abstract]
Net income	$ 419.0	$ 410.4	$ 419.0
Other comprehensive income/(loss):
Foreign currency translation adjustments	24.7	(129.5)
Unrealized (loss)/gain on available-for-sale securities (net of taxes of $nil and $nil)	(0.3)	0.7
Comprehensive income	443.4	$ 281.6
Components of accumulated other comprehensive income
Foreign currency translation adjustments	(157.4)		(182.1)
Unrealized loss on available-for-sale securities, net of taxes	(2.0)		(1.7)
Accumulated other comprehensive loss	$ (159.4)		$ (183.8)