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Fair Value, by Balance Sheet Grouping (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011	Dec. 31, 2010
Financial assets [Abstract]
Cash and cash equivalents	$ 522,638	$ 532,943	$ 520,876	$ 444,250
Securities available for sale	1,118,055	917,722
Held to maturity securities	168,755	216,288
Net loans	2,511,603	2,472,586
Financial liabilities [Abstract]
Short-term borrowings	150,718	147,563
Fair Value Measurements Using Level 1 [Member]
Financial assets [Abstract]
Cash and cash equivalents	522,638
Securities available for sale	10
Held to maturity securities	0
Net loans	0
Accrued interest receivable	0
Financial liabilities [Abstract]
Demand deposits	283,873
Interest bearing deposits	2,294,936
Short-term borrowings	0
Accrued interest payable	106
Fair Value Measurements Using Level 2 [Member]
Financial assets [Abstract]
Cash and cash equivalents	0
Securities available for sale	1,108,413
Held to maturity securities	177,358
Net loans	0
Accrued interest receivable	5,461
Financial liabilities [Abstract]
Demand deposits	0
Interest bearing deposits	1,322,994
Short-term borrowings	150,718
Accrued interest payable	536
Fair Value Measurements Using Level 3 [Member]
Financial assets [Abstract]
Cash and cash equivalents	0
Securities available for sale	0
Held to maturity securities	0
Net loans	2,652,002
Accrued interest receivable	7,447
Financial liabilities [Abstract]
Demand deposits	0
Interest bearing deposits	0
Short-term borrowings	0
Accrued interest payable	0
Carrying Value [Member]
Financial assets [Abstract]
Cash and cash equivalents	522,638	532,943
Securities available for sale	1,108,423	908,718
Held to maturity securities	168,755	216,288
Net loans	2,511,603	2,472,586
Accrued interest receivable	12,908	13,952
Financial liabilities [Abstract]
Demand deposits	283,873	267,776
Interest bearing deposits	3,614,777	3,468,197
Short-term borrowings	150,718	147,563
Accrued interest payable	642	762
Fair Value [Member]
Financial assets [Abstract]
Cash and cash equivalents	522,638	532,943
Securities available for sale	1,108,423	908,718
Held to maturity securities	177,358	224,440
Net loans	2,652,002	2,590,803
Accrued interest receivable	12,908	13,952
Financial liabilities [Abstract]
Demand deposits	283,873	267,776
Interest bearing deposits	3,617,930	3,474,558
Short-term borrowings	150,718	147,563
Accrued interest payable	$ 642	$ 762