Summary of Quarterly Statements of Income (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Data [Abstract]
Interest income	$ 80,629	$ 78,851	$ 69,679	$ 62,630	$ 61,807	$ 56,629	$ 55,180	$ 53,645	$ 291,789	$ 227,261	$ 214,061
Interest Expense	5,153	5,234	3,944	3,450	3,563	3,561	3,720	3,866	17,781	14,710	17,414
Net Interest Income	75,476	73,617	65,735	59,180	58,244	53,068	51,460	49,779	274,008	212,551	196,647
Provision for loan losses	2,000	1,500	1,500	1,500	1,500	1,500	1,500	1,600	6,500	6,100	11,000
Noninterest Income	20,052	20,215	20,674	19,275	18,227	15,351	15,801	13,716	80,216	63,095	60,409
Operating expenses	57,824	58,265	56,785	51,020	52,616	42,702	42,425	42,182	223,894	179,925	164,948
Income before income taxes	35,704	34,067	28,124	25,935	22,355	24,217	23,336	19,713	123,830	89,621	81,108
Federal income tax expense	10,200	9,600	9,100	8,100	7,050	7,450	7,100	5,900	37,000	27,500	24,300
Net Income	$ 25,504	$ 24,467	$ 19,024	$ 17,835	$ 15,305	$ 16,767	$ 16,236	$ 13,813	$ 86,830	$ 62,121	$ 56,808
Basic earnings per common share	$ 0.67	$ 0.64	$ 0.54	$ 0.54	$ 0.47	$ 0.51	$ 0.54	$ 0.46	$ 2.41	$ 1.98	$ 2.02
Diluted earnings per common share	0.66	0.64	0.54	0.54	0.46	0.51	0.54	0.46	2.39	1.97	2.00
Cash dividends declared per share	$ 0.26	$ 0.26	$ 0.24	$ 0.24	$ 0.24	$ 0.24	$ 0.23	$ 0.23	$ 1.00	$ 0.94	$ 0.87