Accumulated Other Comprehensive Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
As at January 1,	$ (31.5)
Net current period other comprehensive (loss)/income	(128.8)
As at March 31,	(160.3)	(31.5)
Foreign currency translation adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
As at January 1,	(25.7)	110.4
Other Comprehensive (loss)/income before reclassification		(1.7)
Gain transferred to the income statement (within Other income),net) on disposal of available-for-sale-securities		0
Net current period other comprehensive (loss)/income	(129.5)	(1.7)
As at March 31,	(155.2)	108.7
Unrealized holding gain/(loss) on available-for-sale securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
As at January 1,	(5.8)	(0.2)
Other Comprehensive (loss)/income before reclassification		7.5
Gain transferred to the income statement (within Other income),net) on disposal of available-for-sale-securities		(3.2)
Net current period other comprehensive (loss)/income	0.7	4.3
As at March 31,	(5.1)	4.1
Accumulated other comprehensive (loss)/income
Accumulated Other Comprehensive Income (Loss) [Line Items]
As at January 1,	(31.5)	110.2
Other Comprehensive (loss)/income before reclassification		5.8
Gain transferred to the income statement (within Other income),net) on disposal of available-for-sale-securities		(3.2)
Net current period other comprehensive (loss)/income	(128.8)	2.6
As at March 31,	$ (160.3)	$ 112.8