Fair Value (Details) (USD $)	3 Months Ended	9 Months Ended																						3 Months Ended	9 Months Ended	12 Months Ended		3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Sep. 30, 2012 Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Sep. 30, 2012 Significant Other Observable Inputs (Level 2) [Member]	Sep. 30, 2012 Significant Unobservable Inputs (Level 3) [Member]	Sep. 30, 2012 Recurring [Member]	Dec. 31, 2011 Recurring [Member]	Sep. 30, 2012 Recurring [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Dec. 31, 2011 Recurring [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Sep. 30, 2012 Recurring [Member] Significant Other Observable Inputs (Level 2) [Member]	Dec. 31, 2011 Recurring [Member] Significant Other Observable Inputs (Level 2) [Member]	Sep. 30, 2012 Recurring [Member] Significant Unobservable Inputs (Level 3) [Member]	Dec. 31, 2011 Recurring [Member] Significant Unobservable Inputs (Level 3) [Member]	Sep. 30, 2012 Nonrecurring [Member]	Dec. 31, 2011 Nonrecurring [Member]	Sep. 30, 2012 Nonrecurring [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Dec. 31, 2011 Nonrecurring [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Sep. 30, 2012 Nonrecurring [Member] Significant Other Observable Inputs (Level 2) [Member]	Dec. 31, 2011 Nonrecurring [Member] Significant Other Observable Inputs (Level 2) [Member]	Sep. 30, 2012 Nonrecurring [Member] Significant Unobservable Inputs (Level 3) [Member]	Dec. 31, 2011 Nonrecurring [Member] Significant Unobservable Inputs (Level 3) [Member]	Sep. 30, 2012 Commercial Real Estate [Member]	Sep. 30, 2012 Commercial Real Estate [Member]	Dec. 31, 2011 Commercial Real Estate [Member]	Dec. 31, 2011 Commercial Real Estate [Member]	Sep. 30, 2012 Residential Real Estate Properties [Member]	Sep. 30, 2012 Residential Real Estate Properties [Member]	Dec. 31, 2011 Residential Real Estate Properties [Member]
Securities available-for sale [Abstract]
U.S. government-sponsored enterprises								$ 427,798,000	$ 563,459,000	$ 0	$ 0	$ 427,798,000	$ 563,459,000	$ 0	$ 0
State and political subdivisions								33,857,000	43,968,000	0	0	33,857,000	43,968,000	0	0
Mortgage-backed securities and collateralized mortgage obligations - residential								515,879,000	204,023,000	0	0	515,879,000	204,023,000	0	0
Small Business Administration-guaranteed participation securities								25,338,000		0		25,338,000		0
Corporate bonds								49,296,000	96,608,000	0	0	49,296,000	96,608,000	0	0
Other securities								660,000	660,000	10,000	10,000	650,000	650,000	0	0
Total Fair Value	1,052,828,000	1,052,828,000		908,718,000	10,000	1,052,818,000	0	1,052,828,000	908,718,000	10,000	10,000	1,052,818,000	908,708,000	0	0
Transfers between level 1 and level 2		0	0
Assets measured at fair value on a non-recurring basis [Abstract]
Other real estate owned																8,691,000	5,265,000	0	0	0	0	8,691,000	5,265,000
Commercial real estate																4,831,000	7,457,000	0	0	0	0	4,831,000	7,457,000
Real estate mortgage 1 to 4 family: first mortgage, impaired																5,649,000	1,732,000	0	0	0	0	5,649,000	1,732,000
Real estate mortgage 1 to 4 family: Home equity loans, impaired																117,000		0		0		117,000
Real estate mortgage 1 to 4 family: Home equity line of credit, impaired																546,000		0		0		546,000
Other real estate																								4,800,000	4,800,000		1,700,000	3,900,000	3,900,000	3,600,000
Valuation charge on other real estate owned	335,000	809,000	3,500,000
Impaired loans	24,600,000	24,600,000		13,700,000
Collateral dependent impaired loans	11,100,000	11,100,000		9,200,000
Gross charge off related to impaired loans																								$ 736,000	$ 2,200,000	$ 1,100,000		$ 1,100,000	$ 1,400,000	$ 1,400,000