Earnings Per 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	$ 9,462	$ 9,535	$ 16,422	$ 15,745
Less: Net income allocated to participating securities	94	83	154	120
Net income available to common shareholders	$ 9,368	$ 9,452	$ 16,268	$ 15,625
Denominator:
Weighted average common shares outstanding	20,785,000	20,338,000	20,723,000	20,319,000
Weighted average dilutive securities (in shares)	612,000	756,000	660,000	779,000
Weighted average common shares outstanding, diluted	21,397,000	21,094,000	21,383,000	21,098,000
Earnings per share
Income per common share, basic (in dollars per share)	$ 0.45	$ 0.46	$ 0.79	$ 0.77
Income per common share, diluted (in dollars per share)	$ 0.44	$ 0.45	$ 0.76	$ 0.74
Employee stock options
Earnings Per Share
Anti-dilutive shares excluded from the diluted earnings per share calculation	221,000	358,000	221,000	112,000
Nonvested performance shares
Earnings Per Share
Anti-dilutive shares excluded from the diluted earnings per share calculation		12,000		12,000