Reportable Operating Segments - Selected Business Segment Financial Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Selected business segment financial information
Net interest income (expense)	$ 573,402	$ 566,318	$ 528,804
Provision for loan and lease losses	(13,800)	(22,180)	(18,500)
Net interest income after provision for loan and lease losses	559,602	544,138	510,304
Noninterest income	192,533	178,993	205,605
Noninterest expense	(370,437)	(364,953)	(347,554)
Income before provision for income taxes	381,698	358,178	368,355
(Provision) benefit for income taxes	(97,306)	(93,784)	(184,673)
Net income (loss)	284,392	264,394	183,682
Assets	20,166,734	20,695,678	20,549,461
Total OTTI write-downs included in earnings		24,085
Gains on sale of real estate			6,922
Retail Banking
Selected business segment financial information
Net interest income (expense)	413,029	420,165	409,796
Provision for loan and lease losses	(6,248)	(8,753)	(6,837)
Net interest income after provision for loan and lease losses	406,781	411,412	402,959
Noninterest income	95,413	89,669	89,372
Noninterest expense	(228,389)	(225,881)	(223,652)
Income before provision for income taxes	273,805	275,200	268,679
(Provision) benefit for income taxes	(69,285)	(70,335)	(130,130)
Net income (loss)	204,520	204,865	138,549
Assets	7,276,047	7,078,016	7,003,724
Commercial Banking
Selected business segment financial information
Net interest income (expense)	141,227	140,333	131,692
Provision for loan and lease losses	(7,552)	(13,427)	(11,663)
Net interest income after provision for loan and lease losses	133,675	126,906	120,029
Noninterest income	73,635	80,904	75,566
Noninterest expense	(82,380)	(80,766)	(68,167)
Income before provision for income taxes	124,930	127,044	127,428
(Provision) benefit for income taxes	(32,298)	(32,700)	(57,857)
Net income (loss)	92,632	94,344	69,571
Assets	6,071,356	6,346,541	5,462,370
Treasury and Other
Selected business segment financial information
Net interest income (expense)	19,146	5,820	(12,684)
Net interest income after provision for loan and lease losses	19,146	5,820	(12,684)
Noninterest income	23,485	8,420	40,667
Noninterest expense	(59,668)	(58,306)	(55,735)
Income before provision for income taxes	(17,037)	(44,066)	(27,752)
(Provision) benefit for income taxes	4,277	9,251	3,314
Net income (loss)	(12,760)	(34,815)	(24,438)
Assets	$ 6,819,331	$ 7,271,121	$ 8,083,367