Other Comprehensive Income/(Loss) (Accumulated other comprehensive income) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Other comprehensive income/(loss) before tax:
Currency translation differences on investments in foreign subsidiaries	$ 97.2	$ (302.9)
Actuarial (loss)/gain related to employee benefit plans, net of tax	(0.4)	0.0
Reclassification of prior service cost/(credit) into employee compensation expense, net of tax	(1.6)	(1.8)
Reclassification of actuarial (gain)/loss into employee compensation expense, net of tax	0.3	0.5
Share of other comprehensive income/(loss) of equity method investments, net of tax	(0.3)	1.1
Unrealized(losses)/gains on available-for-sale investments	2.3	1.2
Reclassification of net (gains)/losses realized on available-for-sale investments included in other gains and losses, net, net of tax	(0.2)	(0.5)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(446.0)	48.8
Other comprehensive income/(loss), net of tax	97.3	(302.4)
Ending balance	(348.7)	(253.6)
Foreign currency translation [Member]
Other comprehensive income/(loss) before tax:
Currency translation differences on investments in foreign subsidiaries	97.2	(302.9)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(365.8)	128.1
Other comprehensive income/(loss), net of tax	97.2	(302.9)
Ending balance	(268.6)	(174.8)
Employee benefit plans [Member]
Other comprehensive income/(loss) before tax:
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, Net of Tax	(0.4)
Reclassification of prior service cost/(credit) into employee compensation expense, net of tax	(1.6)	(1.8)
Reclassification of actuarial (gain)/loss into employee compensation expense, net of tax	0.3	0.5
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(85.6)	(91.7)
Other comprehensive income/(loss), net of tax	(1.7)	(1.3)
Ending balance	(87.3)	(93.0)
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)	1.1
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	5.9	6.5
Other comprehensive income/(loss), net of tax	(0.3)	1.1
Ending balance	5.6	7.6
Available-for-sale investments [Member]
Other comprehensive income/(loss) before tax:
Unrealized(losses)/gains on available-for-sale investments	2.3	1.2
Reclassification of net (gains)/losses realized on available-for-sale investments included in other gains and losses, net, net of tax	(0.2)	(0.5)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(0.5)	5.9
Other comprehensive income/(loss), net of tax	2.1	0.7
Ending balance	$ 1.6	$ 6.6