Quarterly Financial Results (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	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, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Interest income	$ 268,453	$ 275,749	$ 265,851	$ 257,883	$ 256,095	$ 248,110	$ 230,984	$ 211,528	$ 208,085	$ 202,167	$ 123,743	$ 102,143	$ 1,067,936	$ 946,717	$ 636,138
Net interest income	194,172	195,806	188,918	187,246	190,215	189,420	182,236	174,471	174,731	173,182	106,627	88,767	738,859	701,965	519,642
Provision for loan losses	4,644	8,260	7,195	7,184	9,319	8,725	9,402	6,931	6,281	6,920	6,812	3,651	27,283	34,377	23,664
Net income before taxes	118,520	137,224	124,719	117,074	118,757	118,183	109,865	103,143	81,965	99,503	63,074	53,444	497,537	449,948	297,986
Net income	$ 96,079	$ 110,521	$ 100,321	$ 93,960	$ 95,318	$ 93,747	$ 86,865	$ 83,510	$ 26,798	$ 64,442	$ 43,086	$ 39,653	$ 400,881	$ 359,440	$ 173,979
Basic net income per common share (in dollars per share)	$ 1.26	$ 1.45	$ 1.31	$ 1.22	$ 1.24	$ 1.22	$ 1.13	$ 1.08	$ 0.35	$ 0.84	$ 0.81	$ 0.83	$ 5.25	$ 4.66	$ 2.73
Diluted net income per common share (in dollars per share)	$ 1.26	$ 1.44	$ 1.31	$ 1.22	$ 1.23	$ 1.21	$ 1.12	$ 1.08	$ 0.35	$ 0.83	$ 0.80	$ 0.82	$ 5.22	$ 4.64	$ 2.70