Loans and Allowance for Loan Losses (Narrative) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended			9 Months Ended	12 Months Ended			12 Months Ended					12 Months Ended
	Dec. 31, 2012 standard_deviations	Dec. 31, 2011	Dec. 31, 2012 Commercial and Consumer Federal Reserve Bank	Dec. 31, 2011 Commercial and Consumer Federal Reserve Bank	Sep. 30, 2011 Individual Real Estate-Secured Loans	Dec. 31, 2012 Individual Real Estate-Secured Loans	Dec. 31, 2012 Individual Real Estate-Secured Loans Federal Home Loan Bank Advances	Dec. 31, 2011 Individual Real Estate-Secured Loans Federal Home Loan Bank Advances	Dec. 31, 2012 Commercial Real Estate	Dec. 31, 2011 Commercial Real Estate	Dec. 31, 2012 Agricultural Loans	Dec. 31, 2011 Agricultural Loans	Dec. 31, 2011 Commerical Agricultural and Municipal	Dec. 31, 2012 Other Commercial Loans	Dec. 31, 2011 Other Commercial Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, unearned income	$ 736	$ 2,045
Loans pledged as collateral			16,703	18,005			145,064	216,201
Loan participations purchased during period, held for investment									5,871	820	4,849	10,154		16,000	31,144
Transfer of mortgage loans from held-for-sale to loans held for investment					454	8,003
Allowance for loan losses, historical period used for loss rates	5 years	13 years
Standard deviations applied to analysis on estimating loan loss rates	2
Adjustment to historical loan loss rate due to environmental factors (as a percentage)									0.15%	0.10%			0.50%	0.35%
Adjustment to average loss rate due change in historical period used in calculation (as a percentage)										0.27%
Credit commitments available for construction and land development restructured loans	$ 0	$ 356