Summary of Quarterly Statements of Income (Unaudited) (Details) (USD $) In Thousands, except Per 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
Quarterly Financial Data [Abstract]
Interest income	$ 53,126	$ 52,501	$ 52,467	$ 52,664	$ 54,130	$ 53,998	$ 53,439	$ 53,675	$ 210,758	$ 215,242	$ 210,430
Interest Expense	5,132	5,591	6,021	6,469	7,045	7,729	8,145	8,470	23,213	31,389	39,310
Net Interest Income	47,994	46,910	46,446	46,195	47,085	46,269	45,294	45,205	187,545	183,853	171,120
Provision for loan losses	5,000	4,500	4,000	5,000	5,100	6,400	7,000	7,500	18,500	26,000	45,600
Noninterest Income	13,879	12,062	13,282	12,649	11,501	11,225	10,902	10,772	51,872	44,400	42,472
Operating expenses	41,211	36,066	35,537	36,295	37,807	35,394	33,413	35,389	149,109	142,003	136,802
Income before income taxes	15,662	18,406	20,191	17,549	15,679	15,700	15,783	13,088	71,808	60,250	31,190
Federal income tax expense	4,000	5,300	6,325	5,175	4,475	4,075	4,750	3,900	20,800	17,200	8,100
Net Income	$ 11,662	$ 13,106	$ 13,866	$ 12,374	$ 11,204	$ 11,625	$ 11,033	$ 9,188	$ 51,008	$ 43,050	$ 23,090
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
Cash dividends declared per share	$ 0.21	$ 0.21	$ 0.20	$ 0.20	$ 0.20	$ 0.20	$ 0.20	$ 0.20	$ 0.82	$ 0.80	$ 0.80