Unaudited Quarterly Financial Data (Condensed) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Summarized unaudited quarterly financial data
Interest income	$ 87,759	$ 85,944	$ 86,069	$ 84,381	$ 83,211	$ 80,367	$ 78,617	$ 77,486	$ 344,153	$ 319,681	$ 299,919
Interest expense	7,214	7,318	7,424	7,675	7,215	7,309	7,369	7,382	29,631	29,275	26,966
Net interest income	80,545	78,626	78,645	76,706	75,996	73,058	71,248	70,104	314,522	290,406	272,953
Provision for loan losses	1,139	626	0	568	411	826	282	765	2,333	2,284	1,912
Net interest income after provision for loan losses	79,406	78,000	78,645	76,138	75,585	72,232	70,966	69,339	312,189	288,122	271,041
Non-interest income	28,014	28,293	26,759	24,252	24,467	25,799	25,802	22,693	107,318	98,761	90,302
Non-interest expense	66,717	65,180	64,461	62,356	62,203	59,112	59,945	55,497	258,714	236,757	212,679
Income before income taxes	40,703	41,113	40,943	38,034	37,849	38,919	36,823	36,535	160,793	150,126	148,664
Federal and state income tax expense	9,662	10,156	10,492	9,352	8,341	9,305	7,488	8,865	39,662	33,999	35,909
Net Income	$ 31,041	$ 30,957	$ 30,451	$ 28,682	$ 29,508	$ 29,614	$ 29,335	$ 27,670	$ 121,131	$ 116,127	$ 112,755
Basic earnings per share	$ 0.41	$ 0.40	$ 0.40	$ 0.38	$ 0.39	$ 0.39	$ 0.39	$ 0.37	$ 1.59	$ 1.54	$ 1.51
Diluted earnings per share	$ 0.41	$ 0.40	$ 0.40	$ 0.38	$ 0.39	$ 0.39	$ 0.39	$ 0.37	$ 1.59	$ 1.54	$ 1.51