Business Segments - Selected Business Segments Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Business segment financial information
Net Interest Income	$ 124,166			$ 124,896			$ 376,823	$ 373,830
Provision for Credit Losses	28,600			4,250			102,600	11,250
Net Interest Income After Provision for Credit Losses	95,566			120,646			274,223	362,580
Noninterest Income	41,734			46,507			139,151	135,636
Noninterest Expense	(89,949)			(100,349)			(275,153)	(286,131)
Income Before Provision for Income Taxes	47,351			66,804			138,221	212,085
Provision for Income Taxes	(9,511)			(14,752)			(26,731)	(44,315)
Net Income	37,840	$ 38,908	$ 34,742	52,052	$ 56,919	$ 58,799	111,490	167,770
Total Assets	20,109,489			17,672,140			20,109,489	17,672,140	$ 18,095,496
Consumer Banking
Business segment financial information
Net Interest Income	73,868			76,289			221,003	229,704
Provision for Credit Losses	(1,372)			3,116			8,215	7,866
Net Interest Income After Provision for Credit Losses	75,240			73,173			212,788	221,838
Noninterest Income	31,776			36,627			93,309	107,072
Noninterest Expense	(69,408)			(75,198)			(210,744)	(213,168)
Income Before Provision for Income Taxes	37,608			34,602			95,353	115,742
Provision for Income Taxes	(9,627)			(8,506)			(24,235)	(28,745)
Net Income	27,981			26,096			71,118	86,997
Total Assets	7,383,822			6,993,290			7,383,822	6,993,290
Commercial Banking
Business segment financial information
Net Interest Income	47,797			46,791			144,253	139,784
Provision for Credit Losses	(85)			(130)			(819)	1,141
Net Interest Income After Provision for Credit Losses	47,882			46,921			145,072	138,643
Noninterest Income	7,672			8,739			26,483	22,738
Noninterest Expense	(15,430)			(21,330)			(47,552)	(63,545)
Income Before Provision for Income Taxes	40,124			34,330			124,003	97,836
Provision for Income Taxes	(9,784)			(8,560)			(30,278)	(22,507)
Net Income	30,340			25,770			93,725	75,329
Total Assets	5,027,836			4,145,717			5,027,836	4,145,717
Treasury and Other
Business segment financial information
Net Interest Income	2,501			1,816			11,567	4,342
Provision for Credit Losses	30,057			1,264			95,204	2,243
Net Interest Income After Provision for Credit Losses	(27,556)			552			(83,637)	2,099
Noninterest Income	2,286			1,141			19,359	5,826
Noninterest Expense	(5,111)			(3,821)			(16,857)	(9,418)
Income Before Provision for Income Taxes	(30,381)			(2,128)			(81,135)	(1,493)
Provision for Income Taxes	9,900			2,314			27,782	6,937
Net Income	(20,481)			186			(53,353)	5,444
Total Assets	$ 7,697,831			$ 6,533,133			$ 7,697,831	$ 6,533,133