Reportable Operating Segments - Selected Business Segment Financial Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Selected business segment financial information
Net interest income (expense)	$ 528,804	$ 491,672	$ 461,325
Provision for loan and lease losses	(18,500)	(8,600)	(9,900)
Net interest income after provision for loan and lease losses	510,304	483,072	451,425
Noninterest income	205,605	226,037	219,111
Noninterest expense	(347,554)	(337,280)	(327,309)
Income before provision for income taxes	368,355	371,829	343,227
(Provision) benefit for income taxes	(184,673)	(141,651)	(129,447)
Net income	183,682	230,178	213,780
Total assets	20,549,461	19,661,829	19,352,681
Sale of real estate
Gains on sale of real estate	6,922
Retail Banking
Selected business segment financial information
Net interest income (expense)	430,379	415,964	399,153
Provision for loan and lease losses	(6,837)	(3,150)	(4,643)
Net interest income after provision for loan and lease losses	423,542	412,814	394,510
Noninterest income	90,702	91,583	97,934
Noninterest expense	(223,652)	(211,762)	(199,308)
Income before provision for income taxes	290,592	292,635	293,136
(Provision) benefit for income taxes	(138,786)	(110,192)	(99,764)
Net income	151,806	182,443	193,372
Total assets	7,003,724	6,963,701	6,725,665
Commercial Banking
Selected business segment financial information
Net interest income (expense)	111,109	115,455	113,466
Provision for loan and lease losses	(11,663)	(5,450)	(5,257)
Net interest income after provision for loan and lease losses	99,446	110,005	108,209
Noninterest income	74,236	73,264	78,500
Noninterest expense	(68,167)	(63,193)	(61,463)
Income before provision for income taxes	105,515	120,076	125,246
(Provision) benefit for income taxes	(49,201)	(45,172)	(43,181)
Net income	56,314	74,904	82,065
Total assets	5,462,370	4,680,512	4,120,805
Treasury and Other
Selected business segment financial information
Net interest income (expense)	(12,684)	(39,747)	(51,294)
Net interest income after provision for loan and lease losses	(12,684)	(39,747)	(51,294)
Noninterest income	40,667	61,190	42,677
Noninterest expense	(55,735)	(62,325)	(66,538)
Income before provision for income taxes	(27,752)	(40,882)	(75,155)
(Provision) benefit for income taxes	3,314	13,713	13,498
Net income	(24,438)	(27,169)	(61,657)
Total assets	8,083,367	$ 8,017,616	$ 8,506,211
Sale of real estate
Gains on sale of real estate	$ 6,900