Reportable Operating Segments - Selected Business Segment Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Selected business segment financial information
Net interest income (expense)	$ 132,593	$ 134,002	$ 393,232	$ 400,507
Benefit (Provision) for credit losses	4,000	(5,072)	39,000	(101,718)
Net interest income after provision for credit losses	136,593	128,930	432,232	298,789
Noninterest income	50,104	48,898	143,343	143,782
Noninterest expense	(101,036)	(91,629)	(296,730)	(279,545)
Income before provision for income taxes	85,661	86,199	278,845	163,026
(Provision) benefit for income taxes	(21,382)	(21,098)	(70,132)	(39,011)
Net income	64,279	65,101	208,713	124,015
Total assets	25,548,322	22,310,701	25,548,322	22,310,701	$ 22,662,831
Retail Banking
Selected business segment financial information
Net interest income (expense)	97,547	96,709	289,107	278,267
Benefit (Provision) for credit losses	3,067	(2,141)	17,191	(46,517)
Net interest income after provision for credit losses	100,614	94,568	306,298	231,750
Noninterest income	22,698	24,649	68,768	73,197
Noninterest expense	(60,870)	(57,125)	(185,874)	(178,791)
Income before provision for income taxes	62,442	62,092	189,192	126,156
(Provision) benefit for income taxes	(15,660)	(15,452)	(47,670)	(30,529)
Net income	46,782	46,640	141,522	95,627
Total assets	6,881,247	6,947,160	6,881,247	6,947,160
Commercial Banking
Selected business segment financial information
Net interest income (expense)	41,218	38,851	119,718	110,830
Benefit (Provision) for credit losses	4,233	(1,531)	23,728	(47,498)
Net interest income after provision for credit losses	45,451	37,320	143,446	63,332
Noninterest income	18,317	16,679	53,886	52,530
Noninterest expense	(26,371)	(19,914)	(73,842)	(59,481)
Income before provision for income taxes	37,397	34,085	123,490	56,381
(Provision) benefit for income taxes	(9,269)	(8,168)	(30,898)	(13,769)
Net income	28,128	25,917	92,592	42,612
Total assets	6,109,627	6,713,328	6,109,627	6,713,328
Treasury and Other
Selected business segment financial information
Net interest income (expense)	(6,172)	(1,558)	(15,593)	11,410
Benefit (Provision) for credit losses	(3,300)	(1,400)	(1,919)	(7,703)
Net interest income after provision for credit losses	(9,472)	(2,958)	(17,512)	3,707
Noninterest income	9,089	7,570	20,689	18,055
Noninterest expense	(13,795)	(14,590)	(37,014)	(41,273)
Income before provision for income taxes	(14,178)	(9,978)	(33,837)	(19,511)
(Provision) benefit for income taxes	3,547	2,522	8,436	5,287
Net income	(10,631)	(7,456)	(25,401)	(14,224)
Total assets	$ 12,557,448	$ 8,650,213	$ 12,557,448	$ 8,650,213