Reportable Operating Segments - Selected Business Segment Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Selected business segment financial information
Net interest income (expense)	$ 131,254	$ 120,427	$ 260,599	$ 237,739
Provision for loan and lease losses	(4,400)	(1,900)	(8,900)	(2,600)
Net interest income after provision for loan and lease losses	126,854	118,527	251,699	235,139
Noninterest income	48,870	46,371	98,277	119,890
Noninterest expense	(85,241)	(78,473)	(169,580)	(163,537)
Income before provision for income taxes	90,483	86,425	180,396	191,492
(Provision) benefit for income taxes	(33,588)	(31,565)	(66,761)	(71,101)
Net income	56,895	54,860	113,635	120,391
Total assets	20,373,974	19,052,593	20,373,974	19,052,593	$ 19,661,829
Retail Banking
Selected business segment financial information
Net interest income (expense)	106,912	103,771	212,613	206,196
Provision for loan and lease losses	(1,626)	(696)	(3,289)	(952)
Net interest income after provision for loan and lease losses	105,286	103,075	209,324	205,244
Noninterest income	22,381	22,525	46,000	46,155
Noninterest expense	(56,168)	(54,266)	(111,408)	(109,402)
Income before provision for income taxes	71,499	71,334	143,916	141,997
(Provision) benefit for income taxes	(26,485)	(26,067)	(53,218)	(53,055)
Net income	45,014	45,267	90,698	88,942
Total assets	6,779,914	6,841,661	6,779,914	6,841,661
Commercial Banking
Selected business segment financial information
Net interest income (expense)	27,726	28,212	55,259	55,805
Provision for loan and lease losses	(2,774)	(1,204)	(5,611)	(1,648)
Net interest income after provision for loan and lease losses	24,952	27,008	49,648	54,157
Noninterest income	18,921	16,671	36,491	33,019
Noninterest expense	(15,758)	(13,106)	(30,807)	(24,884)
Income before provision for income taxes	28,115	30,573	55,332	62,292
(Provision) benefit for income taxes	(10,409)	(11,145)	(20,443)	(23,241)
Net income	17,706	19,428	34,889	39,051
Total assets	5,408,202	4,460,197	5,408,202	4,460,197
Treasury and Other
Selected business segment financial information
Net interest income (expense)	(3,384)	(11,556)	(7,273)	(24,262)
Net interest income after provision for loan and lease losses	(3,384)	(11,556)	(7,273)	(24,262)
Noninterest income	7,568	7,175	15,786	40,716
Noninterest expense	(13,315)	(11,101)	(27,365)	(29,251)
Income before provision for income taxes	(9,131)	(15,482)	(18,852)	(12,797)
(Provision) benefit for income taxes	3,306	5,647	6,900	5,195
Net income	(5,825)	(9,835)	(11,952)	(7,602)
Total assets	$ 8,185,858	$ 7,750,735	$ 8,185,858	$ 7,750,735