Business Segments (Business Segments Financial Information and Narrative) (Details) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Jun. 30, 2019 USD ($) atm branch	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Business segment financial information
Federal and State effective tax rate used for segment reporting					26.00%
Net Interest Income	$ 124,097		$ 120,496		$ 248,934	$ 239,452
Provision for Credit Losses	4,000		3,500		7,000	7,625
Net Interest Income After Provision for Credit Losses	120,097		116,996		241,934	231,827
Noninterest Income	45,450		41,298		89,129	85,333
Noninterest Expense	(92,725)		(90,791)		(185,782)	(185,175)
Income Before Provision for Income Taxes	72,822		67,503		145,281	131,985
Provision for Income Taxes	(15,903)		(12,785)		(29,563)	(23,227)
Net Income	56,919	$ 58,799	54,718	$ 54,040	115,718	108,758
Total Assets	17,688,845		17,124,162		$ 17,688,845	17,124,162	$ 17,143,974
Retail Banking
Business segment financial information
Number of branch locations through which products and services are delivered to customers | branch					68
Number of ATM's through which products and services are delivered to customers | atm					383
Net Interest Income	67,217		65,683		$ 133,371	130,080
Provision for Credit Losses	2,527		3,445		4,768	7,188
Net Interest Income After Provision for Credit Losses	64,690		62,238		128,603	122,892
Noninterest Income	21,108		19,598		42,341	38,851
Noninterest Expense	(52,086)		(51,939)		(104,610)	(106,538)
Income Before Provision for Income Taxes	33,712		29,897		66,334	55,205
Provision for Income Taxes	(8,231)		(7,473)		(16,336)	(13,764)
Net Income	25,481		22,424		49,998	41,441
Total Assets	6,586,854		6,142,457		6,586,854	6,142,457
Commercial Banking
Business segment financial information
Net Interest Income	45,703		44,010		92,993	86,908
Provision for Credit Losses	(175)		(194)		1,271	(345)
Net Interest Income After Provision for Credit Losses	45,878		44,204		91,722	87,253
Noninterest Income	6,938		5,512		13,999	11,154
Noninterest Expense	(21,260)		(19,858)		(42,215)	(40,190)
Income Before Provision for Income Taxes	31,556		29,858		63,506	58,217
Provision for Income Taxes	(7,945)		(6,740)		(13,947)	(13,564)
Net Income	23,611		23,118		49,559	44,653
Total Assets	4,089,452		3,799,535		4,089,452	3,799,535
Investment Services and Private Banking
Business segment financial information
Net Interest Income	9,846		10,526		20,044	20,413
Provision for Credit Losses	(1)		0		(18)	(60)
Net Interest Income After Provision for Credit Losses	9,847		10,526		20,062	20,473
Noninterest Income	14,859		14,745		28,104	28,415
Noninterest Expense	(16,457)		(16,400)		(33,360)	(32,607)
Income Before Provision for Income Taxes	8,249		8,871		14,806	16,281
Provision for Income Taxes	(2,174)		(2,338)		(3,903)	(4,292)
Net Income	6,075		6,533		10,903	11,989
Total Assets	337,484		342,464		337,484	342,464
Treasury and Other
Business segment financial information
Net Interest Income	1,331		277		2,526	2,051
Provision for Credit Losses	1,649		249		979	842
Net Interest Income After Provision for Credit Losses	(318)		28		1,547	1,209
Noninterest Income	2,545		1,443		4,685	6,913
Noninterest Expense	(2,922)		(2,594)		(5,597)	(5,840)
Income Before Provision for Income Taxes	(695)		(1,123)		635	2,282
Provision for Income Taxes	2,447		3,766		4,623	8,393
Net Income	1,752		2,643		5,258	10,675
Total Assets	$ 6,675,055		$ 6,839,706		$ 6,675,055	$ 6,839,706