Other Comprehensive Income/(Loss) (Accumulated other comprehensive income) (Details) £ in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 GBP (£)
Other comprehensive income/(loss) before tax:
Currency translation differences on investments in foreign subsidiaries, net of tax	$ (63.4)	$ (223.9)	$ (132.8)	$ (364.2)
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)	(1.8)	(5.2)	(4.8)
Reclassification of actuarial (gain)/loss into employee compensation expense, net of tax	0.4	0.6	1.2	1.7
Share of other comprehensive income/(loss) of equity method investments, net of tax	2.4	(1.9)	3.0	(0.6)
Unrealized(losses)/gains on available-for-sale investments	2.3	(5.8)	4.0	(7.6)
Reclassification of net (gains)/losses realized on available-for-sale investments included in other gains and losses, net, net of tax	(0.2)	(0.7)	(0.5)	(1.6)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(513.9)	(94.8)	(446.0)	48.8
Other comprehensive income/(loss), net of tax	(60.3)	(233.5)	(130.7)	(377.1)
Other comprehensive (income)/loss attributable to noncontrolling interests	0.5		3.0
Ending balance	(573.7)	(328.3)	(573.7)	(328.3)
Foreign currency transaction and translation gain			18.3
Foreign currency translation [Member]
Other comprehensive income/(loss) before tax:
Currency translation differences on investments in foreign subsidiaries, net of tax	(63.4)	(223.9)	(132.8)	(364.2)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(432.7)	(12.2)	(365.8)	128.1
Other comprehensive income/(loss), net of tax	(63.4)	(223.9)	(132.8)	(364.2)
Other comprehensive (income)/loss attributable to noncontrolling interests	0.5		3.0
Ending balance	(495.6)	(236.1)	(495.6)	(236.1)
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)	(1.8)	(5.2)	(4.8)
Reclassification of actuarial (gain)/loss into employee compensation expense, net of tax	0.4	0.6	1.2	1.7
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(88.6)	(93.6)	(85.6)	(91.7)
Other comprehensive income/(loss), net of tax	(1.4)	(1.2)	(4.4)	(3.1)
Other comprehensive (income)/loss attributable to noncontrolling interests	0.0		0.0
Ending balance	(90.0)	(94.8)	(90.0)	(94.8)
Equity Method Investments [Member]
Other comprehensive income/(loss) before tax:
Share of other comprehensive income/(loss) of equity method investments, net of tax	2.4
Unrealized(losses)/gains on available-for-sale investments		(1.9)	3.0	(0.6)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	6.5	7.8	5.9	6.5
Other comprehensive income/(loss), net of tax	2.4	(1.9)	3.0	(0.6)
Other comprehensive (income)/loss attributable to noncontrolling interests	0.0		0.0
Ending balance	8.9	5.9	8.9	5.9
Available-for-sale investments [Member]
Other comprehensive income/(loss) before tax:
Unrealized(losses)/gains on available-for-sale investments	2.3	(5.8)	4.0	(7.6)
Reclassification of net (gains)/losses realized on available-for-sale investments included in other gains and losses, net, net of tax	(0.2)	(0.7)	(0.5)	(1.6)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	0.9	3.2	(0.5)	5.9
Other comprehensive income/(loss), net of tax	2.1	(6.5)	3.5	(9.2)
Other comprehensive (income)/loss attributable to noncontrolling interests	0.0		0.0
Ending balance	3.0	$ (3.3)	3.0	$ (3.3)
Designated as Hedging Instrument [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Intercompany debt	$ 168.9		$ 168.9		£ 130