Reportable Operating Segments - Selected Business Segment Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Selected business segment financial information
Net interest income (expense)	$ 127,822	$ 145,613	$ 266,505	$ 290,702
Provision for credit losses	(55,446)	(3,870)	(96,646)	(9,550)
Net interest income after provision for credit losses	72,376	141,743	169,859	281,152
Noninterest income	45,656	48,773	94,884	95,845
Noninterest expense	(91,450)	(93,290)	(187,916)	(185,913)
Income before provision for income taxes	26,582	97,226	76,827	191,084
(Provision) benefit for income taxes	(6,533)	(24,793)	(17,913)	(48,727)
Net income	20,049	72,433	58,914	142,357
Assets	22,993,715	20,526,367	22,993,715	20,526,367	$ 20,166,734
Retail Banking
Selected business segment financial information
Net interest income (expense)	96,146	107,708	186,029	214,935
Provision for credit losses	(24,311)	(1,752)	(44,376)	(4,324)
Net interest income after provision for credit losses	71,835	105,956	141,653	210,611
Noninterest income	22,172	23,375	48,548	47,244
Noninterest expense	(57,639)	(59,495)	(119,283)	(116,662)
Income before provision for income taxes	36,368	69,836	70,918	141,193
(Provision) benefit for income taxes	(9,331)	(17,658)	(16,854)	(35,705)
Net income	27,037	52,178	54,064	105,488
Assets	7,888,155	7,056,325	7,888,155	7,056,325
Commercial Banking
Selected business segment financial information
Net interest income (expense)	33,094	35,000	67,508	69,656
Provision for credit losses	(25,183)	(2,118)	(45,967)	(5,226)
Net interest income after provision for credit losses	7,911	32,882	21,541	64,430
Noninterest income	18,055	18,725	35,851	37,113
Noninterest expense	(20,445)	(19,741)	(41,950)	(39,226)
Income before provision for income taxes	5,521	31,866	15,442	62,317
(Provision) benefit for income taxes	(1,179)	(8,267)	(3,824)	(16,188)
Net income	4,342	23,599	11,618	46,129
Assets	6,006,128	6,525,345	6,006,128	6,525,345
Treasury and Other
Selected business segment financial information
Net interest income (expense)	(1,418)	2,905	12,968	6,111
Provision for credit losses	(5,952)		(6,303)
Net interest income after provision for credit losses	(7,370)	2,905	6,665	6,111
Noninterest income	5,429	6,673	10,485	11,488
Noninterest expense	(13,366)	(14,054)	(26,683)	(30,025)
Income before provision for income taxes	(15,307)	(4,476)	(9,533)	(12,426)
(Provision) benefit for income taxes	3,977	1,132	2,765	3,166
Net income	(11,330)	(3,344)	(6,768)	(9,260)
Assets	$ 9,099,432	$ 6,944,697	$ 9,099,432	$ 6,944,697