Other Comprehensive Income/(Loss) (Accumulated other comprehensive income) (Details) £ in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2016 GBP (£)
Other comprehensive income/(loss) before tax:
Currency translation differences on investments in foreign subsidiaries, net of tax	$ (166.6)	$ 162.6	$ (69.4)	$ (140.3)
Actuarial (loss)/gain related to employee benefit plans, net of tax	0.0	0.0	(0.4)	0.0
Reclassification of prior service cost/(credit) into employee compensation expense, net of tax	(1.8)	(1.2)	(3.4)	(3.0)
Reclassification of actuarial (gain)/loss into employee compensation expense, net of tax	0.5	0.6	0.8	1.1
Share of other comprehensive income/(loss) of equity method investments, net of tax	0.9	0.2	0.6	1.3
Unrealized(losses)/gains on available-for-sale investments	(0.6)	(3.0)	1.7	(1.8)
Reclassification of net (gains)/losses realized on available-for-sale investments included in other gains and losses, net, net of tax	(0.1)	(0.4)	(0.3)	(0.9)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Gain (Loss) Arising During Period, Net of Tax			13.3
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(348.7)	(253.6)	(446.0)	48.8
Other comprehensive income/(loss), net of tax	(167.7)	158.8	(70.4)	(143.6)
Ending balance	(513.9)	(94.8)	(513.9)	(94.8)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest	2.5		2.5
Foreign currency translation [Member]
Other comprehensive income/(loss) before tax:
Currency translation differences on investments in foreign subsidiaries, net of tax	(166.6)	162.6	(69.4)	(140.3)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(268.6)	(174.8)	(365.8)	128.1
Other comprehensive income/(loss), net of tax	(166.6)	162.6	(69.4)	(140.3)
Ending balance	(432.7)	(12.2)	(432.7)	(12.2)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest	2.5		2.5
Employee benefit plans [Member]
Other comprehensive income/(loss) before tax:
Actuarial (loss)/gain related to employee benefit plans, net of tax	0.0		(0.4)
Reclassification of prior service cost/(credit) into employee compensation expense, net of tax	(1.8)	(1.2)	(3.4)	(3.0)
Reclassification of actuarial (gain)/loss into employee compensation expense, net of tax	0.5	0.6	0.8	1.1
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(87.3)	(93.0)	(85.6)	(91.7)
Other comprehensive income/(loss), net of tax	(1.3)	(0.6)	(3.0)	(1.9)
Ending balance	(88.6)	(93.6)	(88.6)	(93.6)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest	0.0		0.0
Equity method investments [Member]
Other comprehensive income/(loss) before tax:
Share of other comprehensive income/(loss) of equity method investments, net of tax	0.9	0.2	0.6	1.3
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	5.6	7.6	5.9	6.5
Other comprehensive income/(loss), net of tax	0.9	0.2	0.6	1.3
Ending balance	6.5	7.8	6.5	7.8
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest	0.0		0.0
Available-for-sale investments [Member]
Other comprehensive income/(loss) before tax:
Unrealized(losses)/gains on available-for-sale investments	(0.6)	(3.0)	1.7	(1.8)
Reclassification of net (gains)/losses realized on available-for-sale investments included in other gains and losses, net, net of tax	(0.1)	(0.4)	(0.3)	(0.9)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	1.6	6.6	(0.5)	5.9
Other comprehensive income/(loss), net of tax	(0.7)	(3.4)	1.4	(2.7)
Ending balance	0.9	$ 3.2	0.9	$ 3.2
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest	0.0		0.0
Designated as Hedging Instrument [Member]
Other comprehensive income/(loss) before tax:
Intercompany Debt	$ 173.9		$ 173.9		£ 130