Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Basic earnings per share:
Net income	$ 10,649	$ 7,028
Weighted-average basic shares	16,769	13,883
Basic earnings per share (in dollars per share)	$ 0.64	$ 0.51
Diluted earnings per share:
Net income	$ 10,649	$ 7,028
Weighted-average basic shares	16,769	13,883
Effect of dilutive securities (in shares)	167	68
Weighted-average dilutive shares	16,936	13,951
Diluted earnings per share (in dollars per share)	$ 0.63	$ 0.50
Earnings Per Share
Antidilutive stock options excluded from the calculation of diluted earnings per share (in shares)	21,361	216,268
Exercise price of outstanding stock options, low end of range (in dollars per share)	$ 41.45	$ 31.10
Exercise price of outstanding stock options, high end of range (in dollars per share)	$ 41.45	$ 40.99