Reportable Operating Segments - Selected Business Segment Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Selected business segment financial information
Net interest income (expense)	$ 141,258	$ 133,319	$ 422,333	$ 393,918
Provision for loan and lease losses	(4,460)	(4,500)	(16,430)	(13,400)
Net interest income after provision for loan and lease losses	136,798	128,819	405,903	380,518
Noninterest income	47,405	49,664	145,902	151,281
Noninterest expense	(93,147)	(84,784)	(275,599)	(257,704)
Income before provision for income taxes	91,056	93,699	276,206	274,095
(Provision) benefit for income taxes	(23,668)	(35,336)	(71,807)	(102,097)
Net income	67,388	58,363	204,399	171,998
Assets	19,983,838	20,565,627	19,983,838	20,565,627	$ 20,549,461
Gain on the sale of bank properties				2,667
Retail Banking
Selected business segment financial information
Net interest income (expense)	111,804	108,885	332,731	321,498
Provision for loan and lease losses	(1,760)	(1,663)	(6,484)	(4,952)
Net interest income after provision for loan and lease losses	110,044	107,222	326,247	316,546
Noninterest income	22,676	22,587	67,566	68,587
Noninterest expense	(58,069)	(53,607)	(170,114)	(165,015)
Income before provision for income taxes	74,651	76,202	223,699	220,118
(Provision) benefit for income taxes	(19,450)	(28,756)	(58,291)	(81,974)
Net income	55,201	47,446	165,408	138,144
Assets	6,884,429	6,868,484	6,884,429	6,868,484
Commercial Banking
Selected business segment financial information
Net interest income (expense)	29,639	28,448	86,320	83,707
Provision for loan and lease losses	(2,700)	(2,837)	(9,946)	(8,448)
Net interest income after provision for loan and lease losses	26,939	25,611	76,374	75,259
Noninterest income	17,801	17,521	59,264	56,552
Noninterest expense	(19,731)	(16,641)	(60,575)	(49,988)
Income before provision for income taxes	25,009	26,491	75,063	81,823
(Provision) benefit for income taxes	(6,435)	(9,993)	(19,329)	(30,436)
Net income	18,574	16,498	55,734	51,387
Assets	6,020,137	5,421,428	6,020,137	5,421,428
Treasury and Other
Selected business segment financial information
Net interest income (expense)	(185)	(4,014)	3,282	(11,287)
Net interest income after provision for loan and lease losses	(185)	(4,014)	3,282	(11,287)
Noninterest income	6,928	9,556	19,072	26,142
Noninterest expense	(15,347)	(14,536)	(44,910)	(42,701)
Income before provision for income taxes	(8,604)	(8,994)	(22,556)	(27,846)
(Provision) benefit for income taxes	2,217	3,413	5,813	10,313
Net income	(6,387)	(5,581)	(16,743)	(17,533)
Assets	$ 7,079,272	8,275,715	$ 7,079,272	8,275,715
Gain on the sale of bank properties		$ 2,700		$ 2,700