Fair Value (Estimated fair value financial instruments (incl. carrying amounts)) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 74,587	$ 168,054
Time deposits in other financial institutions	3,605
Trading securities	694	380
Securities available for sale	8,174	12,811
Securities Held to maturity	0	0
Total securities	8,868	13,191
Loans held for sale	0	0
Loans, net	0	0
Mortgage derivatives	0	0
Short term borrowings	0	0
Other borrowings	0	0
Derivatives	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Demand Deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Savings Deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Time Deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Agricultural Portfolio Segment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Residential Portfolio Segment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Consumer Portfolio Segment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	0	0
Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0	0
Time deposits in other financial institutions	0
Trading securities	0	0
Securities available for sale	1,512,225	1,488,924
Securities Held to maturity	55,846	55,982
Total securities	1,568,071	1,544,906
Loans held for sale	91,762	96,165
Loans, net	2,680,724	2,702,155
Mortgage derivatives	7,319	9,815
Short term borrowings	202,694	224,626
Other borrowings	329,318	376,422
Derivatives	7,062	7,846
Significant Other Observable Inputs (Level 2) | Demand Deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	971,142	974,232
Significant Other Observable Inputs (Level 2) | Savings Deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	2,022,625	2,004,438
Significant Other Observable Inputs (Level 2) | Time Deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	848,689	866,990
Significant Other Observable Inputs (Level 2) | Agricultural Portfolio Segment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	300,908	313,424
Significant Other Observable Inputs (Level 2) | Residential Portfolio Segment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	222,924	230,390
Significant Other Observable Inputs (Level 2) | Consumer Portfolio Segment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	239,437	237,857
Significant Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0	0
Time deposits in other financial institutions	0
Trading securities	0	0
Securities available for sale	4,170	4,340
Securities Held to maturity	0	0
Total securities	4,170	4,340
Loans held for sale	0	0
Loans, net	66,741	78,952
Mortgage derivatives	0	0
Short term borrowings	0	0
Other borrowings	0	0
Derivatives	0	0
Significant Unobservable Inputs (Level 3) | Demand Deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	0	0
Significant Unobservable Inputs (Level 3) | Savings Deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	0	0
Significant Unobservable Inputs (Level 3) | Time Deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	0	0
Significant Unobservable Inputs (Level 3) | Agricultural Portfolio Segment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	13,046	16,019
Significant Unobservable Inputs (Level 3) | Residential Portfolio Segment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	6,509	6,660
Significant Unobservable Inputs (Level 3) | Consumer Portfolio Segment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	3,525	4,121
Commercial | Quoted Prices in Active Markets for Identical Assets (Level 1) | Commercial Portfolio Segment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	0	0
Commercial | Significant Other Observable Inputs (Level 2) | Commercial Portfolio Segment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	669,922	683,924
Commercial | Significant Unobservable Inputs (Level 3) | Commercial Portfolio Segment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	7,431	7,681
Commercial Real Estate | Quoted Prices in Active Markets for Identical Assets (Level 1) | Commercial Portfolio Segment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	0	0
Commercial Real Estate | Significant Other Observable Inputs (Level 2) | Commercial Portfolio Segment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	1,247,533	1,236,560
Commercial Real Estate | Significant Unobservable Inputs (Level 3) | Commercial Portfolio Segment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	36,230	44,471
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	74,587	168,054
Time deposits in other financial institutions	3,605
Trading securities	694	380
Securities available for sale	1,524,569	1,506,075
Securities Held to maturity	55,456	55,502
Total securities	1,580,719	1,561,957
Loans held for sale	91,708	96,165
Loans, net	2,759,106	2,790,087
Mortgage derivatives	7,319	9,815
Short term borrowings	202,694	224,626
Other borrowings	336,577	389,025
Derivatives	7,062	7,846
Carrying Amount | Demand Deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	971,142	974,232
Carrying Amount | Savings Deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	2,022,625	2,004,438
Carrying Amount | Time Deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	848,689	866,990
Carrying Amount | Agricultural Portfolio Segment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	312,810	326,867
Carrying Amount | Residential Portfolio Segment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	240,080	248,608
Carrying Amount | Consumer Portfolio Segment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	239,766	238,617
Carrying Amount | Commercial | Commercial Portfolio Segment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	683,160	700,225
Carrying Amount | Commercial Real Estate | Commercial Portfolio Segment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	1,283,290	1,275,770
Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	74,587	168,054
Time deposits in other financial institutions	3,605
Trading securities	694	380
Securities available for sale	1,524,569	1,506,075
Securities Held to maturity	55,846	55,982
Total securities	1,581,109	1,562,437
Loans held for sale	91,762	96,165
Loans, net	2,747,465	2,781,107
Mortgage derivatives	7,319	9,815
Short term borrowings	202,694	224,626
Other borrowings	329,318	376,422
Derivatives	7,062	7,846
Estimated Fair Value | Demand Deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	971,142	974,232
Estimated Fair Value | Savings Deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	2,022,625	2,004,438
Estimated Fair Value | Time Deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	848,689	866,990
Estimated Fair Value | Agricultural Portfolio Segment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	313,954	329,443
Estimated Fair Value | Residential Portfolio Segment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	229,433	237,050
Estimated Fair Value | Consumer Portfolio Segment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	242,962	241,978
Estimated Fair Value | Commercial | Commercial Portfolio Segment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	677,353	691,605
Estimated Fair Value | Commercial Real Estate | Commercial Portfolio Segment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	$ 1,283,763	$ 1,281,031