Other Comprehensive Income/(Loss) (Accumulated other comprehensive income) (Details) £ in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2017 USD ($)	Mar. 31, 2016 USD ($)	Mar. 31, 2017 GBP (£)
Other comprehensive income/(loss) before tax:
Currency translation differences on investments in foreign subsidiaries, net of tax	$ 62.6	$ 97.2
Actuarial (loss)/gain related to employee benefit plans, net of tax	(0.4)	(0.4)
Reclassification of prior service cost/(credit) into employee compensation expense, net of tax	0.0	(1.6)
Reclassification of actuarial (gain)/loss into employee compensation expense, net of tax	0.6	0.3
Share of other comprehensive income/(loss) of equity method investments, net of tax	0.3	(0.3)
Unrealized(losses)/gains on available-for-sale investments	3.1	2.3
Reclassification of net (gains)/losses realized on available-for-sale investments included in other gains and losses, net, net of tax	(0.6)	(0.2)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(809.3)	(446.0)
Other comprehensive income/(loss), net of tax	65.6	97.3
Ending balance	(743.7)	(348.7)
Foreign currency transaction and translation gain	(1.9)
Foreign currency translation [Member]
Other comprehensive income/(loss) before tax:
Currency translation differences on investments in foreign subsidiaries, net of tax	62.6	97.2
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(679.9)	(365.8)
Other comprehensive income/(loss), net of tax	62.6	97.2
Ending balance	(617.3)	(268.6)
Employee benefit plans [Member]
Other comprehensive income/(loss) before tax:
Actuarial (loss)/gain related to employee benefit plans, net of tax	(0.4)	(0.4)
Reclassification of prior service cost/(credit) into employee compensation expense, net of tax	0.0	(1.6)
Reclassification of actuarial (gain)/loss into employee compensation expense, net of tax	0.6	0.3
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(139.2)	(85.6)
Other comprehensive income/(loss), net of tax	0.2	(1.7)
Ending balance	(139.0)	(87.3)
Available-for-sale investments [Member]
Other comprehensive income/(loss) before tax:
Unrealized(losses)/gains on available-for-sale investments	3.1	2.3
Reclassification of net (gains)/losses realized on available-for-sale investments included in other gains and losses, net, net of tax	(0.6)	(0.2)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	5.0	(0.5)
Other comprehensive income/(loss), net of tax	2.5	2.1
Ending balance	7.5	1.6
Designated as Hedging Instrument [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Intercompany debt	162.5		£ 130.0
Equity Method Investments [Member]
Other comprehensive income/(loss) before tax:
Share of other comprehensive income/(loss) of equity method investments, net of tax	0.3	(0.3)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	4.8	5.9
Other comprehensive income/(loss), net of tax	0.3	(0.3)
Ending balance	$ 5.1	$ 5.6