Other Comprehensive Income/(Loss) (Accumulated other comprehensive income) (Details) £ in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Mar. 31, 2018 GBP (£)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income, Other, Net of Tax	$ (1.6)	$ 3.0
Currency translation differences on investments in foreign subsidiaries, net of tax	64.6	62.6
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(391.2)	(809.3)
Other comprehensive income/(loss), net of tax	63.0	65.6
Ending balance	(331.4)	(743.7)
Foreign currency transaction and translation gain	(6.6)
Foreign currency translation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income, Other, Net of Tax	0.0	0.0
Currency translation differences on investments in foreign subsidiaries, net of tax	64.6	62.6
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(290.5)	(679.9)
Other comprehensive income/(loss), net of tax	64.6	62.6
Ending balance	(225.9)	(617.3)
Employee benefit plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income, Other, Net of Tax	0.4	0.2
Currency translation differences on investments in foreign subsidiaries, net of tax	0.0	0.0
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(109.7)	(139.2)
Other comprehensive income/(loss), net of tax	0.4	0.2
Ending balance	(109.3)	(139.0)
Available-for-sale investments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income, Other, Net of Tax	0.3	2.5
Currency translation differences on investments in foreign subsidiaries, net of tax	0.0	0.0
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	4.7	5.0
Other comprehensive income/(loss), net of tax	0.3	2.5
Ending balance	1.8	7.5
Designated as Hedging Instrument [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Intercompany debt	182.5		£ 130.0
Equity Method Investments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income, Other, Net of Tax	(2.3)	0.3
Currency translation differences on investments in foreign subsidiaries, net of tax	0.0	0.0
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	4.3	4.8
Other comprehensive income/(loss), net of tax	(2.3)	0.3
Ending balance	2.0	$ 5.1
Accounting Standards Update 2016-01 [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Net Income	(3.2)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(394.4)
Accounting Standards Update 2016-01 [Member] | Foreign currency translation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Net Income	0.0
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(290.5)
Accounting Standards Update 2016-01 [Member] | Employee benefit plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Net Income	0.0
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(109.7)
Accounting Standards Update 2016-01 [Member] | Available-for-sale investments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Net Income	(3.2)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	1.5
Accounting Standards Update 2016-01 [Member] | Equity Method Investments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Net Income	0.0
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 4.3