Business Segments (Business Segments Financial Information and Narrative) (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) atm branch	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Segment Reporting [Abstract]
Federal and State Effective Tax Rate Used for Segment Reporting			37.00%
Business segment financial information
Net Interest Income	$ 97,891	$ 95,379	$ 292,443	$ 283,024
Provision for Credit Losses	0	(2,665)	0	(4,864)
Net Interest Income After Provision for Credit Losses	97,891	98,044	292,443	287,888
Noninterest Income	43,221	44,950	141,453	134,199
Noninterest Expense	(91,888)	(81,030)	(262,377)	(245,659)
Income Before Provision for Income Taxes	49,224	61,964	171,519	176,428
Provision for Income Taxes	(14,948)	(20,195)	(53,647)	(54,577)
Net Income	34,276	41,769	117,872	121,851
Total Assets	15,164,123	14,510,166	$ 15,164,123	14,510,166	$ 14,787,208
Retail Banking
Business segment financial information
Number of branch locations through which products and services are delivered to customers | branch			72
Number of ATM's through which products and services are delivered to customers | atm			455
Net Interest Income	51,732	45,432	$ 150,631	130,971
Provision for Credit Losses	2,209	1,853	5,660	2,895
Net Interest Income After Provision for Credit Losses	49,523	43,579	144,971	128,076
Noninterest Income	21,206	20,132	61,123	59,473
Noninterest Expense	(49,963)	(48,569)	(149,461)	(146,581)
Income Before Provision for Income Taxes	20,766	15,142	56,633	40,968
Provision for Income Taxes	(7,352)	(5,498)	(20,097)	(15,248)
Net Income	13,414	9,644	36,536	25,720
Total Assets	4,578,333	3,896,841	4,578,333	3,896,841
Commercial Banking
Business segment financial information
Net Interest Income	37,133	31,780	107,292	86,934
Provision for Credit Losses	(226)	(834)	(956)	(2,164)
Net Interest Income After Provision for Credit Losses	37,359	32,614	108,248	89,098
Noninterest Income	4,777	5,818	16,125	17,617
Noninterest Expense	(25,482)	(16,410)	(59,786)	(49,834)
Income Before Provision for Income Taxes	16,654	22,022	64,587	56,881
Provision for Income Taxes	(5,586)	(7,599)	(22,449)	(19,343)
Net Income	11,068	14,423	42,138	37,538
Total Assets	3,102,549	2,680,761	3,102,549	2,680,761
Investment Services
Business segment financial information
Net Interest Income	4,517	3,864	13,153	11,098
Provision for Credit Losses	(20)	(9)	(36)	(303)
Net Interest Income After Provision for Credit Losses	4,537	3,873	13,189	11,401
Noninterest Income	14,363	14,467	44,770	43,228
Noninterest Expense	(14,031)	(13,616)	(43,193)	(41,379)
Income Before Provision for Income Taxes	4,869	4,724	14,766	13,250
Provision for Income Taxes	(1,802)	(1,751)	(5,463)	(4,912)
Net Income	3,067	2,973	9,303	8,338
Total Assets	232,641	192,739	232,641	192,739
Treasury and Other
Business segment financial information
Net Interest Income	4,509	14,303	21,367	54,021
Provision for Credit Losses	(1,963)	(3,675)	(4,668)	(5,292)
Net Interest Income After Provision for Credit Losses	6,472	17,978	26,035	59,313
Noninterest Income	2,875	4,533	19,435	13,881
Noninterest Expense	(2,412)	(2,435)	(9,937)	(7,865)
Income Before Provision for Income Taxes	6,935	20,076	35,533	65,329
Provision for Income Taxes	(208)	(5,347)	(5,638)	(15,074)
Net Income	6,727	14,729	29,895	50,255
Total Assets	$ 7,250,600	$ 7,739,825	$ 7,250,600	$ 7,739,825