Business Segments (Business Segments Financial Information and Narrative) (Details) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Jun. 30, 2020 USD ($) atm branch	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Business segment financial information
Federal and State effective tax rate used for segment reporting					26.00%
Net Interest Income	$ 126,691		$ 124,097		$ 252,657	$ 248,934
Provision for Credit Losses	40,400		4,000		74,000	7,000
Net Interest Income After Provision for Credit Losses	86,291		120,097		178,657	241,934
Noninterest Income	51,268		45,450		97,417	89,129
Noninterest Expense	(88,892)		(92,725)		(185,204)	(185,782)
Income Before Provision for Income Taxes	48,667		72,822		90,870	145,281
Provision for Income Taxes	(9,759)		(15,903)		(17,220)	(29,563)
Net Income	38,908	$ 34,742	56,919	$ 58,799	73,650	115,718
Total Assets	19,769,942		17,688,845		$ 19,769,942	17,688,845	$ 18,095,496
Consumer Banking
Business segment financial information
Number of branch locations through which products and services are delivered to customers | branch					67
Number of ATM's through which products and services are delivered to customers | atm					367
Net Interest Income	73,221		77,063		$ 147,135	153,415
Provision for Credit Losses	6,137		2,526		9,588	4,750
Net Interest Income After Provision for Credit Losses	67,084		74,537		137,547	148,665
Noninterest Income	28,943		35,967		61,533	70,445
Noninterest Expense	(70,744)		(68,543)		(141,644)	(137,970)
Income Before Provision for Income Taxes	25,283		41,961		57,436	81,140
Provision for Income Taxes	(6,451)		(10,405)		(14,527)	(20,239)
Net Income	18,832		31,556		42,909	60,901
Total Assets	7,416,337		6,924,338		7,416,337	6,924,338
Commercial Banking
Business segment financial information
Net Interest Income	51,219		45,703		96,456	92,993
Provision for Credit Losses	(1,025)		(175)		(735)	1,271
Net Interest Income After Provision for Credit Losses	52,244		45,878		97,191	91,722
Noninterest Income	7,076		6,938		18,811	13,999
Noninterest Expense	(14,776)		(21,260)		(32,122)	(42,215)
Income Before Provision for Income Taxes	44,544		31,556		83,880	63,506
Provision for Income Taxes	(10,940)		(7,945)		(20,494)	(13,947)
Net Income	33,604		23,611		63,386	49,559
Total Assets	5,033,297		4,089,452		5,033,297	4,089,452
Treasury and Other
Business segment financial information
Net Interest Income	2,251		1,331		9,066	2,526
Provision for Credit Losses	35,288		1,649		65,147	979
Net Interest Income After Provision for Credit Losses	(33,037)		(318)		(56,081)	1,547
Noninterest Income	15,249		2,545		17,073	4,685
Noninterest Expense	(3,372)		(2,922)		(11,438)	(5,597)
Income Before Provision for Income Taxes	(21,160)		(695)		(50,446)	635
Provision for Income Taxes	7,632		2,447		17,801	4,623
Net Income	(13,528)		1,752		(32,645)	5,258
Total Assets	$ 7,320,308		$ 6,675,055		$ 7,320,308	$ 6,675,055