Earnings Per Common Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
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	$ 13,106	$ 11,625	$ 39,346	$ 31,846
Denominator for basic earnings per common share, weighted average common shares outstanding	27,498,000	27,457,000	27,490,000	27,454,000
Weighted average common stock equivalents	90,000	47,000	78,000	40,000
Denominator for diluted earnings per common share	27,588,000	27,504,000	27,568,000	27,494,000
Basic earnings per common share	$ 0.48	$ 0.42	$ 1.43	$ 1.16
Diluted earnings per common share	$ 0.48	$ 0.42	$ 1.43	$ 1.16
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	611,605	670,725	618,703	663,492