Common Shares (Details) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	0 Months Ended	3 Months Ended
	Feb. 20, 2015	Mar. 31, 2015	Mar. 31, 2014
Common shares
Issuance of common shares for stock options exercised (in shares)		0.02	0.50
Proceeds from stock options exercised		$ 200,000	$ 4,900,000
Common shares withheld by company		0.6	0.3
Common shares fair value		7,100,000	6,600,000
Common stock dividend declared (in dollars per share)	$ 0.06
Cash dividends paid		$ 17,470,000	$ 11,893,000