Reportable Operating Segments - Selected Business Segment Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Selected business segment financial information
Net interest income (expense)	$ 131,481	$ 127,822	$ 260,639	$ 266,505
Benefit (Provision) for credit losses	35,000	(55,446)	35,000	(96,646)
Net interest income (expense) after provision for credit losses	166,481	72,376	295,639	169,859
Noninterest income	49,371	45,656	93,239	94,884
Noninterest expense	(99,388)	(91,450)	(195,694)	(187,916)
Income before provision for income taxes	116,464	26,582	193,184	76,827
(Provision) benefit for income taxes	(29,723)	(6,533)	(48,750)	(17,913)
Net income (loss)	86,741	20,049	144,434	58,914
Total assets	24,246,328	22,993,715	24,246,328	22,993,715	$ 22,662,831
Retail Banking
Selected business segment financial information
Net interest income (expense)	97,106	91,675	191,560	181,558
Benefit (Provision) for credit losses	12,654	(24,311)	14,124	(44,376)
Net interest income (expense) after provision for credit losses	109,760	67,364	205,684	137,182
Noninterest income	22,495	22,172	46,070	48,548
Noninterest expense	(62,115)	(60,022)	(125,004)	(121,666)
Income before provision for income taxes	70,140	29,514	126,750	64,064
(Provision) benefit for income taxes	(17,884)	(7,554)	(32,010)	(15,077)
Net income (loss)	52,256	21,960	94,740	48,987
Total assets	6,862,294	7,068,251	6,862,294	7,068,251
Commercial Banking
Selected business segment financial information
Net interest income (expense)	40,234	37,565	78,500	71,979
Benefit (Provision) for credit losses	17,465	(25,183)	19,495	(45,967)
Net interest income (expense) after provision for credit losses	57,699	12,382	97,995	26,012
Noninterest income	19,528	18,055	35,569	35,851
Noninterest expense	(24,993)	(18,062)	(47,471)	(39,567)
Income before provision for income taxes	52,234	12,375	86,093	22,296
(Provision) benefit for income taxes	(13,285)	(2,956)	(21,629)	(5,601)
Net income (loss)	38,949	9,419	64,464	16,695
Total assets	6,404,153	6,826,032	6,404,153	6,826,032
Treasury and Other
Selected business segment financial information
Net interest income (expense)	(5,859)	(1,418)	(9,421)	12,968
Benefit (Provision) for credit losses	4,881	(5,952)	1,381	(6,303)
Net interest income (expense) after provision for credit losses	(978)	(7,370)	(8,040)	6,665
Noninterest income	7,348	5,429	11,600	10,485
Noninterest expense	(12,280)	(13,366)	(23,219)	(26,683)
Income before provision for income taxes	(5,910)	(15,307)	(19,659)	(9,533)
(Provision) benefit for income taxes	1,446	3,977	4,889	2,765
Net income (loss)	(4,464)	(11,330)	(14,770)	(6,768)
Total assets	$ 10,979,881	$ 9,099,432	$ 10,979,881	$ 9,099,432