Unaudited Quarterly Financial Data (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
Summarized unaudited quarterly financial data
Interest income	$ 83,211	$ 80,367	$ 78,617	$ 77,486	$ 76,179	$ 75,690	$ 73,963	$ 74,087	$ 319,681	$ 299,919	$ 263,576
Interest expense	7,215	7,309	7,369	7,382	7,368	6,430	6,528	6,640	29,275	26,966	28,758
Net interest income	75,996	73,058	71,248	70,104	68,811	69,260	67,435	67,447	290,406	272,953	234,818
Provision for loan losses	411	826	282	765	191	360	239	1,122	2,284	1,912	6,887
Net interest income after provision for loan losses	75,585	72,232	70,966	69,339	68,620	68,900	67,196	66,325	288,122	271,041	227,931
Non-interest income	24,467	25,799	25,802	22,693	23,978	24,432	22,504	19,388	98,761	90,302	93,047
Non-interest expense	62,203	59,112	59,945	55,497	55,698	54,238	52,673	50,070	236,757	212,679	195,317
Income before income taxes	37,849	38,919	36,823	36,535	36,900	39,094	37,027	35,643	150,126	148,664	125,661
Federal and state income tax expense	8,341	9,305	7,488	8,865	8,846	9,800	8,350	8,913	33,999	35,909	30,017
Net Income	$ 29,508	$ 29,614	$ 29,335	$ 27,670	$ 28,054	$ 29,294	$ 28,677	$ 26,730	$ 116,127	$ 112,755	$ 95,644
Basic earnings per share	$ 0.39	$ 0.39	$ 0.39	$ 0.37	$ 0.37	$ 0.40	$ 0.38	$ 0.36	$ 1.54	$ 1.51	$ 1.31
Diluted earnings per share	$ 0.39	$ 0.39	$ 0.39	$ 0.37	$ 0.37	$ 0.40	$ 0.38	$ 0.36	$ 1.54	$ 1.51	$ 1.31