Allowance for Loan Losses Allowance by impairment analysis method (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Mar. 31, 2011	Dec. 31, 2010
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	$ 86,863		$ 91,906
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	4,266		4,208
Loans and Leases Receivable, Allowance	91,129	99,054	96,114	98,759	98,742	98,403
Financing Receivable, Collectively Evaluated for Impairment	4,872,983		4,561,435
Financing Receivable, Individually Evaluated for Impairment	94,624		89,793
Total loans	4,967,607		4,651,228
Commercial and industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	11,467		12,014
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	123		149
Loans and Leases Receivable, Allowance	11,590	13,243	12,163	12,322	12,460	13,438
Financing Receivable, Collectively Evaluated for Impairment	779,986		671,330
Financing Receivable, Individually Evaluated for Impairment	6,349		6,718
Total loans	786,335		678,048
Commercial real estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	56,755		60,318
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	2,507		3,307
Loans and Leases Receivable, Allowance	59,262	64,577	63,625	65,084	66,914	65,760
Financing Receivable, Collectively Evaluated for Impairment	1,744,506		1,620,109
Financing Receivable, Individually Evaluated for Impairment	60,305		58,165
Total loans	1,804,811		1,678,274
Construction and land [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	4,645		6,163
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	425		219
Loans and Leases Receivable, Allowance	5,070	6,163	6,382	7,769	6,004	6,875
Financing Receivable, Collectively Evaluated for Impairment	159,243		145,937
Financing Receivable, Individually Evaluated for Impairment	5,648		7,772
Total loans	164,891		153,709
Residential [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	9,879		8,884
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	1,080		402
Loans and Leases Receivable, Allowance	10,959	10,844	9,286	9,174	8,499	7,449
Financing Receivable, Collectively Evaluated for Impairment	1,919,386		1,806,722
Financing Receivable, Individually Evaluated for Impairment	21,831		16,681
Total loans	1,941,217		1,823,403
Home equity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	1,298		1,404
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	131		131
Loans and Leases Receivable, Allowance	1,429	1,489	1,535	1,308	1,291	1,231
Financing Receivable, Collectively Evaluated for Impairment	127,761		143,241
Financing Receivable, Individually Evaluated for Impairment	491		457
Total loans	128,252		143,698
Consumer and other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	689		1,149
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	0		0
Loans and Leases Receivable, Allowance	689	740	1,149	1,196	1,616	1,478
Financing Receivable, Collectively Evaluated for Impairment	142,101		174,096
Financing Receivable, Individually Evaluated for Impairment	0		0
Total loans	142,101		174,096
Unallocated Financing Receivables [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	2,130		1,974
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	0		0
Loans and Leases Receivable, Allowance	2,130	1,998	1,974	1,906	1,958	2,172
Financing Receivable, Collectively Evaluated for Impairment	0		0
Financing Receivable, Individually Evaluated for Impairment	0		0
Total loans	$ 0		$ 0