Unaudited Quarterly Financial Data (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
Summarized unaudited quarterly financial data
Interest income	$ 76,179	$ 75,690	$ 73,963	$ 74,087	$ 73,939	$ 69,531	$ 62,151	$ 57,955	$ 299,919	$ 263,576	$ 253,757
Interest expense	7,368	6,430	6,528	6,640	6,929	7,186	7,185	7,458	26,966	28,758	35,714
Net Interest Income	68,811	69,260	67,435	67,447	67,010	62,345	54,966	50,497	272,953	234,818	218,043
Provision for loan losses	191	360	239	1,122	1,802	1,907	1,078	2,100	1,912	6,887	21,525
Net interest income after provision for loan losses	68,620	68,900	67,196	66,325	65,208	60,438	53,888	48,397	271,041	227,931	196,518
Noninterest Income	23,978	24,432	22,504	19,388	23,002	23,873	23,222	22,950	90,302	93,047	91,496
Noninterest Expense	55,698	54,238	52,673	50,070	53,034	50,368	48,481	43,434	212,679	195,317	193,421
Income Before Income Taxes	36,900	39,094	37,027	35,643	35,176	33,943	28,629	27,913	148,664	125,661	94,593
Federal and state income tax expense	8,846	9,800	8,350	8,913	8,630	8,315	5,927	7,145	35,909	30,017	19,077
Net Income	$ 28,054	$ 29,294	$ 28,677	$ 26,730	$ 26,546	$ 25,628	$ 22,702	$ 20,768	$ 112,755	$ 95,644	$ 75,516
Basic earnings per share	$ 0.37	$ 0.40	$ 0.38	$ 0.36	$ 0.36	$ 0.35	$ 0.31	$ 0.29	$ 1.51	$ 1.31	$ 1.05
Diluted earnings per share	$ 0.37	$ 0.40	$ 0.38	$ 0.36	$ 0.36	$ 0.35	$ 0.31	$ 0.29	$ 1.51	$ 1.31	$ 1.05