Business Segments - Selected Business Segment Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Business segment financial information
Net Interest Income	$ 126,819			$ 124,166			$ 370,902	$ 376,823
Provision for Credit Losses	(10,400)			28,600			(40,800)	102,600
Net Interest Income (Loss) After Provision for Credit Losses	137,219			95,566			411,702	274,223
Noninterest Income	41,378			41,734			128,779	139,151
Noninterest Expense	(96,519)			(89,949)			(291,911)	(275,153)
Income (Loss) Before Provision for Income Taxes	82,078			47,351			248,570	138,221
Provision for Income Taxes	(20,025)			(9,511)			(59,035)	(26,731)
Net Income	62,053	$ 67,533	$ 59,949	37,840	$ 38,908	$ 34,742	189,535	111,490
Total Assets	22,965,383			20,109,489			22,965,383	20,109,489	$ 20,603,651
Consumer Banking
Business segment financial information
Net Interest Income	72,062			73,868			212,991	221,003
Provision for Credit Losses	1,235			(1,372)			5,088	8,215
Net Interest Income (Loss) After Provision for Credit Losses	70,827			75,240			207,903	212,788
Noninterest Income	32,046			31,776			98,344	93,309
Noninterest Expense	(71,377)			(69,408)			(222,426)	(210,744)
Income (Loss) Before Provision for Income Taxes	31,496			37,608			83,821	95,353
Provision for Income Taxes	(8,001)			(9,627)			(20,840)	(24,235)
Net Income	23,495			27,981			62,981	71,118
Total Assets	7,530,513			7,383,822			7,530,513	7,383,822
Commercial Banking
Business segment financial information
Net Interest Income	50,088			47,797			146,269	144,253
Provision for Credit Losses	(42)			(85)			205	(819)
Net Interest Income (Loss) After Provision for Credit Losses	50,130			47,882			146,064	145,072
Noninterest Income	7,906			7,672			22,339	26,483
Noninterest Expense	(15,924)			(15,430)			(47,343)	(47,552)
Income (Loss) Before Provision for Income Taxes	42,112			40,124			121,060	124,003
Provision for Income Taxes	(10,373)			(9,784)			(29,634)	(30,278)
Net Income	31,739			30,340			91,426	93,725
Total Assets	5,087,831			5,027,836			5,087,831	5,027,836
Treasury and Other
Business segment financial information
Net Interest Income	4,669			2,501			11,642	11,567
Provision for Credit Losses	(11,593)			30,057			(46,093)	95,204
Net Interest Income (Loss) After Provision for Credit Losses	16,262			(27,556)			57,735	(83,637)
Noninterest Income	1,426			2,286			8,096	19,359
Noninterest Expense	(9,218)			(5,111)			(22,142)	(16,857)
Income (Loss) Before Provision for Income Taxes	8,470			(30,381)			43,689	(81,135)
Provision for Income Taxes	(1,651)			9,900			(8,561)	27,782
Net Income	6,819			(20,481)			35,128	(53,353)
Total Assets	$ 10,347,039			$ 7,697,831			$ 10,347,039	$ 7,697,831