Reportable Operating Segments - Selected Business Segment Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Selected business segment financial information
Net interest income (expense)	$ 133,319	$ 122,683	$ 393,918	$ 360,422
Provision for loan and lease losses	(4,500)	(2,100)	(13,400)	(4,700)
Net interest income after provision for loan and lease losses	128,819	120,583	380,518	355,722
Noninterest income	48,535	48,690	146,812	168,580
Noninterest expense	(83,655)	(82,804)	(253,235)	(246,341)
Income before provision for income taxes	93,699	86,469	274,095	277,961
(Provision) benefit for income taxes	(35,336)	(33,234)	(102,097)	(104,335)
Net income	58,363	53,235	171,998	173,626
Total assets	20,565,627	19,892,693	20,565,627	19,892,693	$ 19,661,829
Sale of bank properties
Gain on the sale of bank properties			2,667
Retail Banking
Selected business segment financial information
Net interest income (expense)	108,885	104,048	321,498	310,244
Provision for loan and lease losses	(1,663)	(770)	(4,952)	(1,722)
Net interest income after provision for loan and lease losses	107,222	103,278	316,546	308,522
Noninterest income	22,587	22,398	68,587	68,553
Noninterest expense	(53,607)	(51,093)	(165,015)	(160,495)
Income before provision for income taxes	76,202	74,583	220,118	216,580
(Provision) benefit for income taxes	(28,756)	(27,672)	(81,974)	(80,727)
Net income	47,446	46,911	138,144	135,853
Total assets	6,868,484	6,850,483	6,868,484	6,850,483
Commercial Banking
Selected business segment financial information
Net interest income (expense)	28,448	28,655	83,707	84,460
Provision for loan and lease losses	(2,837)	(1,330)	(8,448)	(2,978)
Net interest income after provision for loan and lease losses	25,611	27,325	75,259	81,482
Noninterest income	16,786	15,479	53,277	48,498
Noninterest expense	(15,906)	(14,792)	(46,713)	(39,676)
Income before provision for income taxes	26,491	28,012	81,823	90,304
(Provision) benefit for income taxes	(9,993)	(10,399)	(30,436)	(33,640)
Net income	16,498	17,613	51,387	56,664
Total assets	5,421,428	4,655,114	5,421,428	4,655,114
Treasury and Other
Selected business segment financial information
Net interest income (expense)	(4,014)	(10,020)	(11,287)	(34,282)
Net interest income after provision for loan and lease losses	(4,014)	(10,020)	(11,287)	(34,282)
Noninterest income	9,162	10,813	24,948	51,529
Noninterest expense	(14,142)	(16,919)	(41,507)	(46,170)
Income before provision for income taxes	(8,994)	(16,126)	(27,846)	(28,923)
(Provision) benefit for income taxes	3,413	4,837	10,313	10,032
Net income	(5,581)	(11,289)	(17,533)	(18,891)
Total assets	8,275,715	$ 8,387,096	8,275,715	$ 8,387,096
Sale of bank properties
Gain on the sale of bank properties	$ 2,700		$ 2,700