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, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly Financial Data [Abstract]
Interest income	$ 55,323	$ 53,578	$ 52,781	$ 52,379	$ 53,126	$ 52,501	$ 52,467	$ 52,664	$ 214,061	$ 210,758	$ 215,242
Interest Expense	4,018	4,284	4,385	4,727	5,132	5,591	6,021	6,469	17,414	23,213	31,389
Net Interest Income	51,305	49,294	48,396	47,652	47,994	46,910	46,446	46,195	196,647	187,545	183,853
Provision for loan losses	2,000	3,000	3,000	3,000	5,000	4,500	4,000	5,000	11,000	18,500	26,000
Noninterest Income	13,578	14,644	15,948	16,239	14,696	12,719	13,944	13,325	60,409	54,684	46,890
Operating expenses	42,405	39,545	41,041	41,957	42,028	36,723	36,199	36,971	164,948	151,921	144,493
Income before income taxes	20,478	21,393	20,303	18,934	15,662	18,406	20,191	17,549	81,108	71,808	60,250
Federal income tax expense	6,100	6,400	6,100	5,700	4,000	5,300	6,325	5,175	24,300	20,800	17,200
Net Income	$ 14,378	$ 14,993	$ 14,203	$ 13,234	$ 11,662	$ 13,106	$ 13,866	$ 12,374	$ 56,808	$ 51,008	$ 43,050
Basic earnings per common share	$ 0.48	$ 0.54	$ 0.52	$ 0.48	$ 0.42	$ 0.48	$ 0.50	$ 0.45	$ 2.02	$ 1.86	$ 1.57
Diluted earnings per common share	$ 0.48	$ 0.53	$ 0.51	$ 0.48	$ 0.42	$ 0.48	$ 0.50	$ 0.45	$ 2.00	$ 1.85	$ 1.57
Cash dividends declared per share	$ 0.23	$ 0.22	$ 0.21	$ 0.21	$ 0.21	$ 0.21	$ 0.20	$ 0.20	$ 0.87	$ 0.82	$ 0.80