Earnings Per Common Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Numerator for both basic and diluted earnings per common share:
Net income	$ 11,998	$ 11,918	$ 23,988	$ 22,159
Denominator:
Total average basic common shares outstanding	26,647,050	26,610,450	26,637,537	26,599,791
Effect of dilutive stock options	3,275	959	3,342	1,297
Total diluted average common shares outstanding	26,650,325	26,611,409	26,640,879	26,601,088
Earnings per common share - basic	$ 0.45	$ 0.45	$ 0.90	$ 0.83
Earnings per common share - diluted	$ 0.45	$ 0.45	$ 0.90	$ 0.83