Reportable Operating Segments - Selected Business Segment Financial Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Selected business segment financial information
Net interest income	$ 535,734	$ 573,402	$ 566,318
Provision for credit losses	(121,718)	(13,800)	(22,180)
Net interest income (loss) after provision for credit losses	414,016	559,602	544,138
Noninterest income	197,380	192,533	178,993
Noninterest expense	(367,672)	(370,437)	(364,953)
Income (loss) before (provision) benefit for income taxes	243,724	381,698	358,178
(Provision) benefit for income taxes	(57,970)	(97,306)	(93,784)
Net income (loss)	185,754	284,392	264,394
Assets	22,662,831	20,166,734	20,695,678
Total OTTI write-downs included in earnings			24,085
Retail Banking
Selected business segment financial information
Net interest income	393,466	413,029	420,165
Provision for credit losses	(52,719)	(6,248)	(8,753)
Net interest income (loss) after provision for credit losses	340,747	406,781	411,412
Noninterest income	98,026	95,413	89,669
Noninterest expense	(231,404)	(228,389)	(225,881)
Income (loss) before (provision) benefit for income taxes	207,369	273,805	275,200
(Provision) benefit for income taxes	(48,605)	(69,285)	(70,335)
Net income (loss)	158,764	204,520	204,865
Assets	7,611,375	7,276,047	7,078,016
Commercial Banking
Selected business segment financial information
Net interest income	133,301	141,227	140,333
Provision for credit losses	(53,921)	(7,552)	(13,427)
Net interest income (loss) after provision for credit losses	79,380	133,675	126,906
Noninterest income	78,202	73,635	80,904
Noninterest expense	(81,533)	(82,380)	(80,766)
Income (loss) before (provision) benefit for income taxes	76,049	124,930	127,044
(Provision) benefit for income taxes	(17,171)	(32,298)	(32,700)
Net income (loss)	58,878	92,632	94,344
Assets	5,810,090	6,071,356	6,346,541
Treasury and Other
Selected business segment financial information
Net interest income	8,967	19,146	5,820
Provision for credit losses	(15,078)
Net interest income (loss) after provision for credit losses	(6,111)	19,146	5,820
Noninterest income	21,152	23,485	8,420
Noninterest expense	(54,735)	(59,668)	(58,306)
Income (loss) before (provision) benefit for income taxes	(39,694)	(17,037)	(44,066)
(Provision) benefit for income taxes	7,806	4,277	9,251
Net income (loss)	(31,888)	(12,760)	(34,815)
Assets	$ 9,241,366	$ 6,819,331	$ 7,271,121