Accumulated Other Comprehensive Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013
Accumulated Other Comprehensive Income (Loss)
As at January 1, 2013	$ 86.9
Current period change	(37.8)
As at March 31, 2013	49.1
Foreign currency translation adjustment
Accumulated Other Comprehensive Income (Loss)
As at January 1, 2013	85.1
Current period change	(36.1)
As at March 31, 2013	49.0
Unrealized holding gain/(loss) on available-for-sale securities
Accumulated Other Comprehensive Income (Loss)
As at January 1, 2013	1.8
Current period change	(1.7)
As at March 31, 2013	$ 0.1