Accumulated Other Comprehensive Income (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]
As at January 1, 2014	$ 110.2
Other comprehensive income, net of tax	(141.7)	23.3	26.6
As at December 31, 2014	(31.5)	110.2
Foreign currency translation adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
As at January 1, 2014	110.4
Other Comprehensive income before reclassification	(136.1)
Gain transferred to the income statement (within Other income/(expense),net) on disposal of available-for-sale-securities	0
Other comprehensive income, net of tax	(136.1)
As at December 31, 2014	(25.7)
Unrealized holding gain/(loss) on available-for-sale securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
As at January 1, 2014	(0.2)
Other Comprehensive income before reclassification	3.7
Gain transferred to the income statement (within Other income/(expense),net) on disposal of available-for-sale-securities	(9.3)
Other comprehensive income, net of tax	(5.6)
As at December 31, 2014	(5.8)
Accumulated other comprehensive income
Accumulated Other Comprehensive Income (Loss) [Line Items]
As at January 1, 2014	110.2
Other Comprehensive income before reclassification	(132.4)
Gain transferred to the income statement (within Other income/(expense),net) on disposal of available-for-sale-securities	(9.3)
Other comprehensive income, net of tax	(141.7)	23.3	26.6
As at December 31, 2014	$ (31.5)	$ 110.2