Fair Value of Assets and Liabilities (Details 3) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Recurring basis
Total (Gains) / Losses for assets measured at fair value on a non-recurring basis
Assets transfer from Level 1 to level 2	$ 0		$ 0
Assets transfer from Level 2 to level 1	0		0
Nonrecurring basis
Total (Gains) / Losses for assets measured at fair value on a non-recurring basis
Total (Gains) / Losses	25	6,231
Assets transfer from Level 1 to level 2	0		0
Assets transfer from Level 2 to level 1	0		0
Nonrecurring basis | Commercial and industrial
Total (Gains) / Losses for assets measured at fair value on a non-recurring basis
Total (Gains) / Losses on impaired loans	44	1,941
Nonrecurring basis | Agriculture
Total (Gains) / Losses for assets measured at fair value on a non-recurring basis
Total (Gains) / Losses on impaired loans		28
Nonrecurring basis | Construction
Total (Gains) / Losses for assets measured at fair value on a non-recurring basis
Total (Gains) / Losses on impaired loans		460
Nonrecurring basis | Land
Total (Gains) / Losses for assets measured at fair value on a non-recurring basis
Total (Gains) / Losses on impaired loans	(19)	3,802
Nonrecurring basis | Significant Other Observable Inputs (Level 2)
Assets
Total assets measured on a non-recurring basis		2,548
Nonrecurring basis | Significant Other Observable Inputs (Level 2) | Commercial and industrial
Assets
Impaired loans		820
Nonrecurring basis | Significant Other Observable Inputs (Level 2) | Agriculture
Assets
Impaired loans		72
Nonrecurring basis | Significant Other Observable Inputs (Level 2) | Construction
Assets
Impaired loans		1,656
Nonrecurring basis | Significant Unobservable Inputs (Level 3)
Assets
Total assets measured on a non-recurring basis	2,174	2,261
Nonrecurring basis | Significant Unobservable Inputs (Level 3) | Commercial and industrial
Assets
Impaired loans	181	213
Nonrecurring basis | Significant Unobservable Inputs (Level 3) | Land
Assets
Impaired loans	1,993	2,048
Nonrecurring basis | Assets At Fair Value
Assets
Total assets measured on a non-recurring basis	2,174	4,809
Nonrecurring basis | Assets At Fair Value | Commercial and industrial
Assets
Impaired loans	181	1,033
Nonrecurring basis | Assets At Fair Value | Agriculture
Assets
Impaired loans		72
Nonrecurring basis | Assets At Fair Value | Construction
Assets
Impaired loans		1,656
Nonrecurring basis | Assets At Fair Value | Land
Assets
Impaired loans	$ 1,993	$ 2,048