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, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Data [Abstract]
Interest income	$ 61,807	$ 56,629	$ 55,180	$ 53,645	$ 55,323	$ 53,578	$ 52,781	$ 52,379	$ 227,261	$ 214,061	$ 210,758
Interest Expense	3,563	3,561	3,720	3,866	4,018	4,284	4,385	4,727	14,710	17,414	23,213
Net Interest Income	58,244	53,068	51,460	49,779	51,305	49,294	48,396	47,652	212,551	196,647	187,545
Provision for loan losses	1,500	1,500	1,500	1,600	2,000	3,000	3,000	3,000	6,100	11,000	18,500
Noninterest Income	18,227	15,351	15,801	13,716	13,578	14,644	15,948	16,239	63,095	60,409	54,684
Operating expenses	52,616	42,702	42,425	42,182	42,405	39,545	41,041	41,957	179,925	164,948	151,921
Income before income taxes	22,355	24,217	23,336	19,713	20,478	21,393	20,303	18,934	89,621	81,108	71,808
Federal income tax expense	7,050	7,450	7,100	5,900	6,100	6,400	6,100	5,700	27,500	24,300	20,800
Net Income	$ 15,305	$ 16,767	$ 16,236	$ 13,813	$ 14,378	$ 14,993	$ 14,203	$ 13,234	$ 62,121	$ 56,808	$ 51,008
Basic earnings per common share	$ 0.47	$ 0.51	$ 0.54	$ 0.46	$ 0.48	$ 0.54	$ 0.52	$ 0.48	$ 1.98	$ 2.02	$ 1.86
Diluted earnings per common share	$ 0.46	$ 0.51	$ 0.54	$ 0.46	$ 0.48	$ 0.53	$ 0.51	$ 0.48	$ 1.97	$ 2.00	$ 1.85
Cash dividends declared per share	$ 0.24	$ 0.24	$ 0.23	$ 0.23	$ 0.23	$ 0.22	$ 0.21	$ 0.21	$ 0.94	$ 0.87	$ 0.82