Unaudited Quarterly Financial Data (Condensed) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Summarized unaudited quarterly financial data
Interest income	$ 125,310	$ 122,905	$ 117,715	$ 103,066	$ 96,898	$ 96,464	$ 94,032	$ 87,628	$ 468,996	$ 375,022	$ 344,153
Interest expense	9,436	9,160	9,161	7,774	7,072	7,652	7,774	7,366	35,531	29,864	29,631
Net interest income	115,874	113,745	108,554	95,292	89,826	88,812	86,258	80,262	433,465	345,158	314,522
Provision for loan losses	1,246	3,194	4,718	795	2,886	3,327	3,013	1,598	9,953	10,824	2,333
Net interest income after provision for loan losses	114,628	110,551	103,836	94,497	86,940	85,485	83,245	78,664	423,512	334,334	312,189
Non-interest income	28,494	32,416	31,828	26,086	27,709	31,185	27,656	25,689	118,824	112,239	107,318
Non-interest expense	81,876	82,829	81,795	73,627	68,366	68,552	65,309	63,344	320,127	265,571	258,714
Income before income taxes	61,246	60,138	53,869	46,956	46,283	48,118	45,592	41,009	222,209	181,002	160,793
Federal and state income tax expense	11,647	10,802	9,485	8,397	31,327	11,639	11,905	9,754	40,331	64,625	39,662
Net Income	$ 49,599	$ 49,336	$ 44,384	$ 38,559	$ 14,956	$ 36,479	$ 33,687	$ 31,255	$ 181,878	$ 116,377	$ 121,131
Basic earnings per share	$ 0.59	$ 0.58	$ 0.53	$ 0.48	$ 0.19	$ 0.47	$ 0.43	$ 0.41	$ 2.18	$ 1.50	$ 1.59
Diluted earnings per share	$ 0.59	$ 0.58	$ 0.52	$ 0.48	$ 0.19	$ 0.47	$ 0.43	$ 0.41	$ 2.17	$ 1.50	$ 1.59