Fair Value (Details) (USD $)	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2012	Jun. 30, 2012	Dec. 31, 2011
Securities available-for sale [Abstract]
Total securities available-for-sale	$ 1,118,055,000	$ 1,118,055,000	$ 917,722,000
Assets measured at fair value on a non-recurring basis [Abstract]
Impaired loans	19,000,000	19,000,000	13,700,000
Recurring [Member]
Securities available-for sale [Abstract]
U.S. government-sponsored enterprises	643,189,000	643,189,000	563,459,000
State and political subdivisions	36,978,000	36,978,000	43,968,000
Mortgage-backed securities and collateralized mortgage obligations - residential	354,285,000	354,285,000	204,023,000
Corporate bonds	73,311,000	73,311,000	96,608,000
Other securities	660,000	660,000	660,000
Total securities available-for-sale	1,108,423,000	1,108,423,000	908,718,000
Transfers between level 1 and level 2		0	0
Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Securities available-for sale [Abstract]
U.S. government-sponsored enterprises	0	0	0
State and political subdivisions	0	0	0
Mortgage-backed securities and collateralized mortgage obligations - residential	0	0	0
Corporate bonds	0	0	0
Other securities	10,000	10,000	10,000
Total securities available-for-sale	10,000	10,000	10,000
Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member]
Securities available-for sale [Abstract]
U.S. government-sponsored enterprises	643,189,000	643,189,000	563,459,000
State and political subdivisions	36,978,000	36,978,000	43,968,000
Mortgage-backed securities and collateralized mortgage obligations - residential	354,285,000	354,285,000	204,023,000
Corporate bonds	73,311,000	73,311,000	96,608,000
Other securities	650,000	650,000	650,000
Total securities available-for-sale	1,108,413,000	1,108,413,000	908,708,000
Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Securities available-for sale [Abstract]
U.S. government-sponsored enterprises	0	0	0
State and political subdivisions	0	0	0
Mortgage-backed securities and collateralized mortgage obligations - residential	0	0	0
Corporate bonds	0	0	0
Other securities	0	0	0
Total securities available-for-sale	0	0	0
Nonrecurring [Member]
Assets measured at fair value on a non-recurring basis [Abstract]
Other real estate owned	3,870,000	3,870,000	5,265,000
Nonrecurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets measured at fair value on a non-recurring basis [Abstract]
Other real estate owned	0	0	0
Nonrecurring [Member] | Significant Other Observable Inputs (Level 2) [Member]
Assets measured at fair value on a non-recurring basis [Abstract]
Other real estate owned	0	0	0
Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Assets measured at fair value on a non-recurring basis [Abstract]
Other real estate owned	3,870,000	3,870,000	5,265,000
Valuation charge on other real estate owned	181,000	474,000	3,500,000
Collateral dependent impaired loans	9,900,000	9,900,000	9,200,000
Commercial real estate [Member]
Assets measured at fair value on a non-recurring basis [Abstract]
Gross charge off related to impaired loans	1,000,000	1,500,000	1,100,000
Commercial real estate [Member] | Nonrecurring [Member]
Assets measured at fair value on a non-recurring basis [Abstract]
Commercial real estate	7,716,000	7,716,000	7,457,000
Commercial real estate [Member] | Nonrecurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets measured at fair value on a non-recurring basis [Abstract]
Commercial real estate	0	0	0
Commercial real estate [Member] | Nonrecurring [Member] | Significant Other Observable Inputs (Level 2) [Member]
Assets measured at fair value on a non-recurring basis [Abstract]
Commercial real estate	0	0	0
Commercial real estate [Member] | Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Assets measured at fair value on a non-recurring basis [Abstract]
Commercial real estate	7,716,000	7,716,000	7,457,000
Real estate mortgage - 1 to 4 family [Member]
Assets measured at fair value on a non-recurring basis [Abstract]
Gross charge off related to impaired loans	233,000	295,000	1,400,000
Real estate mortgage - 1 to 4 family [Member] | First mortgages [Member] | Nonrecurring [Member]
Assets measured at fair value on a non-recurring basis [Abstract]
Real estate mortgage 1 to 4 family: first mortgage, impaired	2,186,000	2,186,000	1,732,000
Real estate mortgage - 1 to 4 family [Member] | First mortgages [Member] | Nonrecurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets measured at fair value on a non-recurring basis [Abstract]
Real estate mortgage 1 to 4 family: first mortgage, impaired	0	0	0
Real estate mortgage - 1 to 4 family [Member] | First mortgages [Member] | Nonrecurring [Member] | Significant Other Observable Inputs (Level 2) [Member]
Assets measured at fair value on a non-recurring basis [Abstract]
Real estate mortgage 1 to 4 family: first mortgage, impaired	0	0	0
Real estate mortgage - 1 to 4 family [Member] | First mortgages [Member] | Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Assets measured at fair value on a non-recurring basis [Abstract]
Real estate mortgage 1 to 4 family: first mortgage, impaired	2,186,000	2,186,000	1,732,000
Real estate mortgage - 1 to 4 family [Member] | Home Equity Loans [Member] | Nonrecurring [Member]
Assets measured at fair value on a non-recurring basis [Abstract]
Real estate mortgage 1 to 4 family: Home equity loans, impaired	33,000	33,000
Real estate mortgage - 1 to 4 family [Member] | Home Equity Loans [Member] | Nonrecurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets measured at fair value on a non-recurring basis [Abstract]
Real estate mortgage 1 to 4 family: Home equity loans, impaired	0	0
Real estate mortgage - 1 to 4 family [Member] | Home Equity Loans [Member] | Nonrecurring [Member] | Significant Other Observable Inputs (Level 2) [Member]
Assets measured at fair value on a non-recurring basis [Abstract]
Real estate mortgage 1 to 4 family: Home equity loans, impaired	0	0
Real estate mortgage - 1 to 4 family [Member] | Home Equity Loans [Member] | Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Assets measured at fair value on a non-recurring basis [Abstract]
Real estate mortgage 1 to 4 family: Home equity loans, impaired	33,000	33,000
Commercial Real Estate [Member] | Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Assets measured at fair value on a non-recurring basis [Abstract]
Other real estate	28,000	28,000	1,700,000
Residential Real Estate Properties [Member] | Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Assets measured at fair value on a non-recurring basis [Abstract]
Other real estate	$ 3,800,000	$ 3,800,000	$ 3,600,000