Common Shares (Details) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	3 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2013	Jun. 30, 2012
Common shares
Vested options exercised by employees to acquire common shares, shares		0.3	1.0
Proceeds from stock options exercised		$ 3,200,000	$ 15,900,000
Amount used to repurchase surrendered unexercised vested options surrendered to satisfy exercise price and related tax withholding obligations		21,000,000
Common shares withheld by company		0.2	0.1
Common shares fair value		3,000,000	2,100,000
Dividend declared (in dollars per share)	$ 0.04
Dividend paid		$ 23,552,000