Allowance for Loan Losses and Credit Quality of Loans, Allowance for Loan Losses (Details) (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance as of Beginning of Period	$ 69,434,000
Balance as of Ending of Period	69,434,000
Allowance for loan losses for acquired loan portfolio	0	0
Provision expense and net charge-offs related to acquired loans	200,000	200,000
Allowance loan losses [Abstract]
Allowance for loan losses	69,434,000
Allowance for loans individually evaluated for impairment	925,000		715,000
Allowance for loans collectively evaluated for impairment	68,509,000		68,719,000
Ending balance of loans	5,482,025,000		5,406,795,000
Commercial Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance as of Beginning of Period	35,090,000	35,624,000
Charge-offs	(479,000)	(3,322,000)
Recoveries	399,000	467,000
Provision	(573,000)	2,589,000
Balance as of Ending of Period	34,437,000	35,358,000
Allowance loan losses [Abstract]
Allowance for loan losses	34,437,000	35,358,000
Allowance for loans individually evaluated for impairment	925,000		715,000
Allowance for loans collectively evaluated for impairment	33,512,000		34,375,000
Ending balance of loans	2,436,202,000		2,392,621,000
Consumer Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance as of Beginning of Period	27,694,000	27,162,000
Charge-offs	(4,032,000)	(3,723,000)
Recoveries	741,000	977,000
Provision	4,033,000	1,869,000
Balance as of Ending of Period	28,436,000	26,285,000
Allowance loan losses [Abstract]
Allowance for loan losses	28,436,000	26,285,000
Allowance for loans individually evaluated for impairment	0		0
Allowance for loans collectively evaluated for impairment	28,436,000		27,694,000
Ending balance of loans	1,989,030,000		1,972,537,000
Residential Real Estate Mortgages [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance as of Beginning of Period	6,520,000	6,252,000
Charge-offs	(319,000)	(671,000)
Recoveries	94,000	14,000
Provision	(70,000)	1,113,000
Balance as of Ending of Period	6,225,000	6,708,000
Allowance loan losses [Abstract]
Allowance for loan losses	6,225,000	6,708,000
Allowance for loans individually evaluated for impairment	0		0
Allowance for loans collectively evaluated for impairment	6,225,000		6,520,000
Ending balance of loans	1,056,793,000		1,041,637,000
Unallocated [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance as of Beginning of Period	130,000	296,000
Charge-offs	0	0
Recoveries	0	0
Provision	206,000	87,000
Balance as of Ending of Period	336,000	383,000
Allowance loan losses [Abstract]
Allowance for loan losses	336,000	383,000
Allowance for loans collectively evaluated for impairment	336,000		130,000
Originated Loans [Member]
Allowance loan losses [Abstract]
Ending balance of loans	4,586,927,000		4,476,403,000
Ending balance of loans individually evaluated for impairment	21,995,000		21,380,000
Ending balance of loans collectively evaluated for impairment	4,564,932,000		4,455,023,000
Originated Loans [Member] | Commercial Loans [Member]
Allowance loan losses [Abstract]
Ending balance of loans individually evaluated for impairment	15,025,000		16,120,000
Ending balance of loans collectively evaluated for impairment	2,025,635,000		1,974,112,000
Originated Loans [Member] | Consumer Loans [Member]
Allowance loan losses [Abstract]
Ending balance of loans individually evaluated for impairment	4,248,000		3,248,000
Ending balance of loans collectively evaluated for impairment	1,785,586,000		1,749,702,000
Originated Loans [Member] | Residential Real Estate Mortgages [Member]
Allowance loan losses [Abstract]
Ending balance of loans individually evaluated for impairment	2,722,000		2,012,000
Ending balance of loans collectively evaluated for impairment	753,711,000		731,209,000
Acquired Loans [Member]
Allowance loan losses [Abstract]
Ending balance of loans	895,098,000		930,392,000
Ending balance of loans individually evaluated for impairment	9,873,000		10,060,000
Ending balance of loans collectively evaluated for impairment	885,225,000		920,332,000
Acquired Loans [Member] | Commercial Loans [Member]
Allowance loan losses [Abstract]
Ending balance of loans individually evaluated for impairment	9,873,000		10,060,000
Ending balance of loans collectively evaluated for impairment	385,669,000		392,329,000
Acquired Loans [Member] | Consumer Loans [Member]
Allowance loan losses [Abstract]
Ending balance of loans individually evaluated for impairment	0		0
Ending balance of loans collectively evaluated for impairment	199,196,000		219,587,000
Acquired Loans [Member] | Residential Real Estate Mortgages [Member]
Allowance loan losses [Abstract]
Ending balance of loans individually evaluated for impairment	0		0
Ending balance of loans collectively evaluated for impairment	$ 300,360,000		$ 308,416,000