Reportable Operating Segments - Selected Business Segment Financial Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Selected business segment financial information
Net interest income (expense)	$ 491,672	$ 461,325	$ 443,798
Provision for loan and lease losses	(8,600)	(9,900)	(11,100)
Net interest income after provision for loan and lease losses	483,072	451,425	432,698
Noninterest income	217,601	211,403	209,237
Noninterest expense	(328,844)	(319,601)	(297,691)
Income before provision for income taxes	371,829	343,227	344,244
Provision for income taxes	(141,651)	(129,447)	(127,572)
Net income	230,178	213,780	216,672
Total assets	19,661,829	19,352,681	18,133,696
Retail Banking
Selected business segment financial information
Net interest income (expense)	415,964	399,153	384,065
Provision for loan and lease losses	(3,150)	(4,643)	(5,249)
Net interest income after provision for loan and lease losses	412,814	394,510	378,816
Noninterest income	91,583	97,934	96,023
Noninterest expense	(211,762)	(199,308)	(186,322)
Income before provision for income taxes	292,635	293,136	288,517
Provision for income taxes	(110,192)	(99,764)	(103,080)
Net income	182,443	193,372	185,437
Total assets	6,963,701	6,725,665	6,271,341
Commercial Banking
Selected business segment financial information
Net interest income (expense)	115,455	113,466	114,188
Provision for loan and lease losses	(5,450)	(5,257)	(5,851)
Net interest income after provision for loan and lease losses	110,005	108,209	108,337
Noninterest income	66,316	72,218	65,319
Noninterest expense	(56,245)	(55,181)	(49,692)
Income before provision for income taxes	120,076	125,246	123,964
Provision for income taxes	(45,172)	(43,181)	(44,169)
Net income	74,904	82,065	79,795
Total assets	4,680,512	4,120,805	3,878,005
Treasury and Other
Selected business segment financial information
Net interest income (expense)	(39,747)	(51,294)	(54,455)
Net interest income after provision for loan and lease losses	(39,747)	(51,294)	(54,455)
Noninterest income	59,702	41,251	47,895
Noninterest expense	(60,837)	(65,112)	(61,677)
Income before provision for income taxes	(40,882)	(75,155)	(68,237)
Provision for income taxes	13,713	13,498	19,677
Net income	(27,169)	(61,657)	(48,560)
Total assets	$ 8,017,616	$ 8,506,211	$ 7,984,350