Summary of Quarterly Statements of Income (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	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, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Selected Quarterly Financial Information [Abstract]
Interest income	$ 169,162	$ 164,944	$ 155,133	$ 142,896	$ 144,416	$ 103,562	$ 82,937	$ 79,464	$ 80,629	$ 78,851	$ 69,679	$ 62,630	$ 632,135	$ 410,379	$ 291,789
Interest expense	23,257	21,316	17,185	12,799	11,969	6,753	5,442	5,134	5,153	5,234	3,944	3,450	74,557	29,298	17,781
Net interest income	145,905	143,628	137,948	130,097	132,447	96,809	77,495	74,330	75,476	73,617	65,735	59,180	557,578	381,081	274,008
Provision for loan losses	7,522	5,499	6,229	4,050	6,272	4,103	3,000	1,500	2,000	1,500	1,500	1,500	23,300	14,875	6,500
Noninterest income	32,319	32,122	41,568	38,010	54,264	27,770	20,897	19,419	20,052	20,215	20,674	19,275	144,019	122,350	80,216
Operating expenses	100,022	119,539	98,237	104,196	114,302	106,144	59,085	58,887	57,824	58,265	56,785	51,020	421,994	338,418	223,894
Income before income taxes	70,680	50,712	75,050	59,861	66,137	14,332	36,307	33,362	35,704	34,067	28,124	25,935	256,303	150,138	123,830
Income tax expense	61,234	10,253	23,036	12,257	18,969	2,848	10,532	9,757	10,200	9,600	9,100	8,100	106,780	42,106	37,000
Net income	$ 9,446	$ 40,459	$ 52,014	$ 47,604	$ 47,168	$ 11,484	$ 25,775	$ 23,605	$ 25,504	$ 24,467	$ 19,024	$ 17,835	$ 149,523	$ 108,032	$ 86,830
Basic earnings per common share (usd per share)	$ 0.13	$ 0.57	$ 0.73	$ 0.67	$ 0.67	$ 0.23	$ 0.67	$ 0.61	$ 0.67	$ 0.64	$ 0.54	$ 0.54	$ 2.11	$ 2.21	$ 2.41
Diluted earnings per common share (usd per share)	0.13	0.56	0.73	0.67	$ 0.66	0.23	0.67	0.60	0.66	0.64	0.54	0.54	2.08	2.17	2.39
Cash dividends declared per share (in dollars per share)	$ 0.27	$ 0.28	$ 0.27	$ 0.27		$ 0.27	$ 0.26	$ 0.26	$ 0.26	$ 0.26	$ 0.24	$ 0.24	$ 1.10	$ 1.06	$ 1.0