Reportable Operating Segments - Selected Business Segment Financial Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Selected business segment financial information
Net interest income (expense)	$ 566,318	$ 528,804	$ 491,672
Provision for loan and lease losses	(22,180)	(18,500)	(8,600)
Net interest income after provision for loan and lease losses	544,138	510,304	483,072
Noninterest income	178,993	205,605	226,037
Noninterest expense	(364,953)	(347,554)	(337,280)
Income (loss) before (provision) benefit for income taxes	358,178	368,355	371,829
(Provision) benefit for income taxes	(93,784)	(184,673)	(141,651)
Net income (loss)	264,394	183,682	230,178
Assets	20,695,678	20,549,461	19,661,829
Total OTTI write-downs included in earnings	24,085
Gain on the sale of bank properties		6,922
Retail Banking
Selected business segment financial information
Net interest income (expense)	447,023	430,379	415,964
Provision for loan and lease losses	(8,753)	(6,837)	(3,150)
Net interest income after provision for loan and lease losses	438,270	423,542	412,814
Noninterest income	90,800	90,702	91,583
Noninterest expense	(225,879)	(223,652)	(211,762)
Income (loss) before (provision) benefit for income taxes	303,191	290,592	292,635
(Provision) benefit for income taxes	(77,872)	(138,786)	(110,192)
Net income (loss)	225,319	151,806	182,443
Assets	7,078,016	7,003,724	6,963,701
Commercial Banking
Selected business segment financial information
Net interest income (expense)	113,475	111,109	115,455
Provision for loan and lease losses	(13,427)	(11,663)	(5,450)
Net interest income after provision for loan and lease losses	100,048	99,446	110,005
Noninterest income	79,773	74,236	73,264
Noninterest expense	(80,766)	(68,167)	(63,193)
Income (loss) before (provision) benefit for income taxes	99,055	105,515	120,076
(Provision) benefit for income taxes	(25,164)	(49,201)	(45,172)
Net income (loss)	73,891	56,314	74,904
Assets	6,346,541	5,462,370	4,680,512
Treasury and Other
Selected business segment financial information
Net interest income (expense)	5,820	(12,684)	(39,747)
Net interest income after provision for loan and lease losses	5,820	(12,684)	(39,747)
Noninterest income	8,420	40,667	61,190
Noninterest expense	(58,308)	(55,735)	(62,325)
Income (loss) before (provision) benefit for income taxes	(44,068)	(27,752)	(40,882)
(Provision) benefit for income taxes	9,252	3,314	13,713
Net income (loss)	(34,816)	(24,438)	(27,169)
Assets	$ 7,271,121	$ 8,083,367	$ 8,017,616