Common Stock (Details) (USD $)	12 Months Ended			0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Feb. 15, 2008
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award
Antidilutive stock options excluded from computation of earnings per share (in shares)	47,067	23,258	60,400
Employee benefit stock
Share-based Compensation Arrangement by Share-based Payment Award
Common share purchase right dividend (in shares)				1
Number of common shares each right is entitled to (in shares)				1
Common share purchase right exercise price (in dollars per share)				$ 200.00