Earnings per Common Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Numerator:
Net income:	$ 20,155	$ 2,397	$ 22,845	$ 2,987
Denominator:
Weighted-average common shares outstanding - basic	12,463,406	12,378,371	12,461,010	12,373,143
Dilutive effect of stock-based compensation plans (in shares)	107,699	128,330	97,424	134,416
Weighted-average common shares outstanding - diluted	12,571,105	12,506,701	12,558,434	12,507,559
Earnings per share
Net income per share - basic (in dollars per share)	$ 1.62	$ 0.19	$ 1.83	$ 0.24
Net income per share - diluted (in dollars per share)	$ 1.60	$ 0.19	$ 1.82	$ 0.24
Stock options
Antidilutive securities excluded from computation of diluted earnings per share
Antidilutive securities excluded from computation of diluted earnings per share (in shares)	349,900	372,000	349,900	372,000