Reportable Operating Segments - Selected Business Segment Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Selected business segment financial information
Net interest income	$ 145,613	$ 141,403	$ 290,702	$ 281,075
Provision for loan and lease losses	(3,870)	(6,020)	(9,550)	(11,970)
Net interest income after provision for loan and lease losses	141,743	135,383	281,152	269,105
Noninterest income	48,773	49,797	95,845	98,497
Noninterest expense	(93,290)	(91,865)	(185,913)	(182,452)
Income before provision for income taxes	97,226	93,315	191,084	185,150
(Provision) benefit for income taxes	(24,793)	(24,262)	(48,727)	(48,139)
Net income	72,433	69,053	142,357	137,011
Retail Banking
Selected business segment financial information
Net interest income	114,127	111,273	228,030	220,927
Provision for loan and lease losses	(1,752)	(2,376)	(4,324)	(4,724)
Net interest income after provision for loan and lease losses	112,375	108,897	223,706	216,203
Noninterest income	23,682	22,158	47,858	44,890
Noninterest expense	(59,492)	(55,584)	(116,658)	(112,045)
Income before provision for income taxes	76,565	75,471	154,906	149,048
(Provision) benefit for income taxes	(19,470)	(19,634)	(39,397)	(38,841)
Net income	57,095	55,837	115,509	110,207
Commercial Banking
Selected business segment financial information
Net interest income	28,581	28,802	56,561	56,681
Provision for loan and lease losses	(2,118)	(3,644)	(5,226)	(7,246)
Net interest income after provision for loan and lease losses	26,463	25,158	51,335	49,435
Noninterest income	18,418	21,463	36,499	41,463
Noninterest expense	(19,741)	(21,196)	(39,226)	(40,844)
Income before provision for income taxes	25,140	25,425	48,608	50,054
(Provision) benefit for income taxes	(6,456)	(6,566)	(12,497)	(12,894)
Net income	18,684	18,859	36,111	37,160
Treasury and Other
Selected business segment financial information
Net interest income	2,905	1,328	6,111	3,467
Net interest income after provision for loan and lease losses	2,905	1,328	6,111	3,467
Noninterest income	6,673	6,176	11,488	12,144
Noninterest expense	(14,057)	(15,085)	(30,029)	(29,563)
Income before provision for income taxes	(4,479)	(7,581)	(12,430)	(13,952)
(Provision) benefit for income taxes	1,133	1,938	3,167	3,596
Net income	$ (3,346)	$ (5,643)	$ (9,263)	$ (10,356)