Business Segments (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Segment Reporting [Abstract]
Federal and State Effective Tax Rate Used for Segment Reporting	37.00%
Business segment financial information
Net Interest Income	$ 96,770	$ 93,233
Provision for Loan and Lease Losses	0	0
Net Interest Income After Provision for Credit Losses	96,770	93,233
Noninterest Income	52,307	44,768
Noninterest Expense	(86,915)	(83,547)
Income Before Provision for Income Taxes	62,162	54,454
Provision for Income Taxes	(19,720)	(15,862)
Net Income	42,442	38,592
Total Assets	15,139,179	14,263,118	14,787,208
Retail Banking
Business segment financial information
Number of branch locations through which products and services are delivered to customers	74
Number of ATM's through which products and services are delivered to customers	456
Net Interest Income	48,015	41,102
Provision for Loan and Lease Losses	1,723	1,456
Net Interest Income After Provision for Credit Losses	46,292	39,646
Noninterest Income	19,073	19,320
Noninterest Expense	(50,033)	(49,096)
Income Before Provision for Income Taxes	15,332	9,870
Provision for Income Taxes	(5,447)	(3,652)
Net Income	9,885	6,218
Total Assets	4,239,641	3,679,909
Commercial Banking
Business segment financial information
Net Interest Income	35,927	28,237
Provision for Loan and Lease Losses	(464)	(61)
Net Interest Income After Provision for Credit Losses	36,391	28,298
Noninterest Income	5,599	6,260
Noninterest Expense	(18,188)	(17,418)
Income Before Provision for Income Taxes	23,802	17,140
Provision for Income Taxes	(8,402)	(5,879)
Net Income	15,400	11,261
Total Assets	2,910,258	2,512,523
Investment Services
Business segment financial information
Net Interest Income	2,977	2,582
Provision for Loan and Lease Losses	(8)	(68)
Net Interest Income After Provision for Credit Losses	2,985	2,650
Noninterest Income	14,717	14,343
Noninterest Expense	(14,444)	(14,235)
Income Before Provision for Income Taxes	3,258	2,758
Provision for Income Taxes	(1,205)	(1,020)
Net Income	2,053	1,738
Total Assets	188,399	183,381
Treasury and Other
Business segment financial information
Net Interest Income	9,851	21,312
Provision for Loan and Lease Losses	(1,251)	(1,327)
Net Interest Income After Provision for Credit Losses	11,102	22,639
Noninterest Income	12,918	4,845
Noninterest Expense	(4,250)	(2,798)
Income Before Provision for Income Taxes	19,770	24,686
Provision for Income Taxes	(4,666)	(5,311)
Net Income	15,104	19,375
Total Assets	$ 7,800,881	$ 7,887,305