Accumulated Other Comprehensive (Loss) Income - Tax Effects on Each Component of Other Comprehensive Income Recognized in Statements of Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Foreign currency cumulative translation adjustments, Before Tax	$ 396.8	$ (128.2)	$ (305.2)
Unrealized cash flow hedge gains, Before Tax	(116.0)	29.7	59.1
Reclassification adjustments on foreign currency hedges, Before Tax	(4.6)	(19.6)	(121.0)
Unrealized gains/(losses) on securities, Before Tax		0.5	(0.2)
Adjustments to prior service cost and unrecognized actuarial assumptions, Before Tax	6.6	27.3	(25.0)
Total Other Comprehensive Income (Loss), Before Tax	282.8	(90.3)	(392.3)
Foreign currency cumulative translation adjustments, Tax	(48.2)	1.8
Unrealized cash flow hedge gains, Tax	(21.0)	1.4	6.4
Reclassification adjustments on foreign currency hedges, Tax	(0.8)	6.2	(28.0)
Adjustments to prior service cost and unrecognized actuarial assumptions, Tax	2.0	5.3	(3.6)
Total Other Comprehensive Income (Loss), Tax	(68.0)	14.7	(25.2)
Foreign currency cumulative translation adjustments, net of tax	445.0	(130.0)	(305.2)
Unrealized cash flow hedge gains, net of tax	(95.0)	28.3	52.7
Reclassification adjustments on cash flow hedges, net of tax	(3.8)	(25.8)	(93.0)
Unrealized gains/(losses) on securities, net of tax		0.5	(0.2)
Adjustments to prior service cost and unrecognized actuarial assumptions, net of tax	4.6	22.0	(21.4)
Total Other Comprehensive Income (Loss)	$ 350.8	$ (105.0)	$ (367.1)