Business Segments (Business Segments Financial Information and Narrative) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($) atm branch	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Business segment financial information
Federal and State effective tax rate used for segment reporting	26.00%
Net Interest Income	$ 125,966	$ 124,837
Provision for Credit Losses	33,600	3,000
Net Interest Income After Provision for Credit Losses	92,366	121,837
Noninterest Income	46,149	43,679
Noninterest Expense	(96,312)	(93,057)
Income Before Provision for Income Taxes	42,203	72,459
Provision for Income Taxes	(7,461)	(13,660)
Net Income	34,742	58,799
Total Assets	$ 18,542,233	17,446,413	$ 18,095,496
Consumer
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	382
Net Interest Income	$ 73,661	76,352
Provision for Credit Losses	3,545	2,224
Net Interest Income After Provision for Credit Losses	70,116	74,128
Noninterest Income	32,590	34,478
Noninterest Expense	(70,900)	(69,427)
Income Before Provision for Income Taxes	31,806	39,179
Provision for Income Taxes	(7,984)	(9,834)
Net Income	23,822	29,345
Total Assets	7,385,185	6,796,106
Commercial
Business segment financial information
Net Interest Income	45,986	47,290
Provision for Credit Losses	290	1,446
Net Interest Income After Provision for Credit Losses	45,696	45,844
Noninterest Income	11,735	7,061
Noninterest Expense	(17,298)	(20,955)
Income Before Provision for Income Taxes	40,133	31,950
Provision for Income Taxes	(9,760)	(6,002)
Net Income	30,373	25,948
Total Assets	4,584,040	4,004,176
Treasury and Other
Business segment financial information
Net Interest Income	6,319	1,195
Provision for Credit Losses	29,765	(670)
Net Interest Income After Provision for Credit Losses	(23,446)	1,865
Noninterest Income	1,824	2,140
Noninterest Expense	(8,114)	(2,675)
Income Before Provision for Income Taxes	(29,736)	1,330
Provision for Income Taxes	10,283	2,176
Net Income	(19,453)	3,506
Total Assets	$ 6,573,008	$ 6,646,131