Earnings Per Common Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Numerator and denominator of the basic and diluted earnings per common share computations
Numerator for both basic and diluted earnings per common share, net income	$ 11,662	$ 13,106	$ 13,866	$ 12,374	$ 11,204	$ 11,625	$ 11,033	$ 9,188	$ 51,008	$ 43,050	$ 23,090
Denominator for basic earnings per common share, weighted average common shares outstanding									27,492,000	27,455,000	26,276,000
Weighted average common stock equivalents									91,000	51,000	29,000
Denominator for diluted earnings per common share									27,583,000	27,506,000	26,305,000
Basic earnings per common share	$ 0.42	$ 0.48	$ 0.50	$ 0.45	$ 0.41	$ 0.42	$ 0.40	$ 0.33	$ 1.86	$ 1.57	$ 0.88
Diluted earnings per common share	$ 0.42	$ 0.48	$ 0.50	$ 0.45	$ 0.41	$ 0.42	$ 0.40	$ 0.33	$ 1.85	$ 1.57	$ 0.88
Stock Option [Member]
Earnings Per Common Share (Textual) [Abstract]
Average number of exercisable employee stock option awards that were not included in the computation of diluted earnings per common share									614,152	660,553	597,710