Summary of Quarterly Statements of Income (Unaudited) (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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Selected Quarterly Financial Information [Abstract]
Interest income	$ 210,103	$ 198,377	$ 189,582	$ 177,934	$ 169,162	$ 164,944	$ 155,133	$ 142,896	$ 144,416	$ 103,562	$ 82,937	$ 79,464	$ 775,996	$ 632,135	$ 410,379
Interest expense	46,651	38,896	32,045	26,071	23,257	21,316	17,185	12,799	11,969	6,753	5,442	5,134	143,663	74,557	29,298
Net interest income	163,452	159,481	157,537	151,863	145,905	143,628	137,948	130,097	132,447	96,809	77,495	74,330	632,333	557,578	381,081
Provision for loan losses	8,894	6,028	9,572	6,256	7,522	5,499	6,229	4,050	6,272	4,103	3,000	1,500	30,750	23,300	14,875
Noninterest income	32,047	37,917	38,018	40,554	32,319	32,122	41,568	38,010	54,264	27,770	20,897	19,419	148,536	144,019	122,350
Operating expenses	108,366	109,661	104,561	101,610	100,022	119,539	98,237	104,196	114,302	106,144	59,085	58,887	424,198	421,994	338,418
Income before income taxes	78,239	81,709	81,422	84,551	70,680	50,712	75,050	59,861	66,137	14,332	36,307	33,362	325,921	256,303	150,138
Income tax expense	5,200	11,312	12,434	12,955	61,234	10,253	23,036	12,257	18,969	2,848	10,532	9,757	41,901	106,780	42,106
Net income	$ 73,039	$ 70,397	$ 68,988	$ 71,596	$ 9,446	$ 40,459	$ 52,014	$ 47,604	$ 47,168	$ 11,484	$ 25,775	$ 23,605	$ 284,020	$ 149,523	$ 108,032
Basic earnings per common share (usd per share)	$ 1.02	$ 0.99	$ 0.97	$ 1.01	$ 0.13	$ 0.57	$ 0.73	$ 0.67	$ 0.67	$ 0.23	$ 0.67	$ 0.61	$ 3.98	$ 2.11	$ 2.21
Diluted earnings per common share (usd per share)	1.01	0.98	0.96	0.99	0.13	0.56	0.73	0.67	0.66	0.23	0.67	0.60	3.94	2.08	2.17
Cash dividends declared per share (in dollars per share)	$ 0.34	$ 0.34	$ 0.28	$ 0.28	$ 0.28	$ 0.28	$ 0.27	$ 0.27	$ 0.27	$ 0.27	$ 0.26	$ 0.26	$ 1.24	$ 1.10	$ 1.06