Quarterly Financial Results (unaudited) (Details) - USD ($)	3 Months Ended												12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	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, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Interest income	$ 257,047,000	$ 249,188,000	$ 251,738,000	$ 263,069,000	$ 268,453,000	$ 275,749,000	$ 265,851,000	$ 257,883,000	$ 256,095,000	$ 248,110,000	$ 230,984,000	$ 211,528,000	$ 1,021,042,000	$ 1,067,936,000	$ 946,717,000
Net interest income	220,985,000	206,594,000	200,657,000	193,552,000	194,172,000	195,806,000	188,918,000	187,246,000	190,215,000	189,420,000	182,236,000	174,471,000	821,788,000	766,142,000	736,342,000
Provision for credit losses	7,180,000	16,333,000	68,332,000	99,889,000	4,644,000	8,260,000	7,195,000	7,184,000	9,319,000	8,725,000	9,402,000	6,931,000	191,734,000	27,283,000	34,377,000
Net income before taxes	133,944,000	137,049,000	73,674,000	26,691,000	118,520,000	137,224,000	124,719,000	117,074,000	118,757,000	118,183,000	109,865,000	103,143,000	371,358,000	497,537,000	449,948,000
Net income	110,876,000	110,645,000	62,444,000	28,356,000									312,321,000	400,881,000	359,440,000
Net income available to common shareholders	$ 107,078,000	$ 106,847,000	$ 62,444,000	$ 28,356,000	$ 96,079,000	$ 110,521,000	$ 100,321,000	$ 93,960,000	$ 95,318,000	$ 93,747,000	$ 86,865,000	$ 83,510,000	$ 304,725,000	$ 400,881,000	$ 359,440,000
Basic net income per common share (in dollars per share)	$ 1.42	$ 1.42	$ 0.83	$ 0.37	$ 1.26	$ 1.45	$ 1.31	$ 1.22	$ 1.24	$ 1.22	$ 1.13	$ 1.08	$ 4.04	$ 5.25	$ 4.66
Diluted net income per common share (in dollars per share)	$ 1.42	$ 1.42	$ 0.83	$ 0.37	$ 1.26	$ 1.44	$ 1.31	$ 1.22	$ 1.23	$ 1.21	$ 1.12	$ 1.08	$ 4.03	$ 5.22	$ 4.64