Reportable Operating Segments - Selected Business Segment Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Selected business segment financial information
Net interest income (expense)	$ 134,002	$ 143,081	$ 400,507	$ 433,783
Provision for credit losses	(5,072)		(101,718)	(9,550)
Net interest income after provision for credit losses	128,930	143,081	298,789	424,233
Noninterest income	48,898	49,980	143,782	145,825
Noninterest expense	(91,629)	(93,466)	(279,545)	(279,379)
Income (loss) before (provision) benefit for income taxes	86,199	99,595	163,026	290,679
(Provision) benefit for income taxes	(21,098)	(25,396)	(39,011)	(74,123)
Net income (loss)	65,101	74,199	124,015	216,556
Assets	22,310,701	20,598,220	22,310,701	20,598,220	$ 20,166,734
Retail Banking
Selected business segment financial information
Net interest income (expense)	102,803	104,977	288,832	319,912
Provision for credit losses	(2,141)		(46,517)	(4,324)
Net interest income after provision for credit losses	100,662	104,977	242,315	315,588
Noninterest income	24,649	23,948	73,197	71,192
Noninterest expense	(57,451)	(57,276)	(176,734)	(173,938)
Income (loss) before (provision) benefit for income taxes	67,860	71,649	138,778	212,842
(Provision) benefit for income taxes	(16,898)	(18,097)	(33,752)	(53,802)
Net income (loss)	50,962	53,552	105,026	159,040
Assets	7,772,539	7,072,484	7,772,539	7,072,484
Commercial Banking
Selected business segment financial information
Net interest income (expense)	32,757	35,911	100,265	105,567
Provision for credit losses	(1,531)		(47,498)	(5,226)
Net interest income after provision for credit losses	31,226	35,911	52,767	100,341
Noninterest income	16,679	17,447	52,530	54,560
Noninterest expense	(19,588)	(20,073)	(61,538)	(59,299)
Income (loss) before (provision) benefit for income taxes	28,317	33,285	43,759	95,602
(Provision) benefit for income taxes	(6,722)	(8,626)	(10,546)	(24,814)
Net income (loss)	21,595	24,659	33,213	70,788
Assets	5,887,949	6,088,667	5,887,949	6,088,667
Treasury and Other
Selected business segment financial information
Net interest income (expense)	(1,558)	2,193	11,410	8,304
Provision for credit losses	(1,400)		(7,703)
Net interest income after provision for credit losses	(2,958)	2,193	3,707	8,304
Noninterest income	7,570	8,585	18,055	20,073
Noninterest expense	(14,590)	(16,117)	(41,273)	(46,142)
Income (loss) before (provision) benefit for income taxes	(9,978)	(5,339)	(19,511)	(17,765)
(Provision) benefit for income taxes	2,522	1,327	5,287	4,493
Net income (loss)	(7,456)	(4,012)	(14,224)	(13,272)
Assets	$ 8,650,213	$ 7,437,069	$ 8,650,213	$ 7,437,069