Related Party Transactions Other Related Party (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended							12 Months Ended			15 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Sep. 30, 2012
Related Party Transaction [Line Items]
Guarantee fees	$ 6,568	$ 6,401	$ 6,064	$ 5,930	$ 5,966	$ 6,320	$ 6,387	$ 24,963	$ 24,821	$ 24,091	$ 6,148
First Dakota Bank [Member]
Related Party Transaction [Line Items]
Guarantee fees										200
Bath State Bank [Member]
Related Party Transaction [Line Items]
Guarantee fees										100
Commitment fees [Member] | AgGeorgia [Member] [Member]
Related Party Transaction [Line Items]
Guarantee fees								100
Mortgage Loans on Real Estate [Member] | First Dakota Bank [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Purchases from Related Party								37,100
USDA Guaranteed Securities [Member] | Bath State Bank [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Purchases from Related Party								4,400	10,600	4,600
Mortgage Loans on Real Estate [Member] | First Dakota Bank [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Purchases from Related Party									29,700	27,000
Long-Term Standby Purchase Commitments [Member] | AgGeorgia [Member] [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Purchases from Related Party								51,500
Preferred Stock [Member] | Fixed Interest Rate [Member]
Related Party Transaction [Line Items]
Available-for-sale Securities, Amortized Cost Basis	79,284				79,662			79,284	79,662
Preferred Stock [Member] | Fixed Interest Rate [Member] | CoBank, ACB [Member]
Related Party Transaction [Line Items]
Available-for-sale Securities, Amortized Cost Basis	78,500							78,500
GSE Subordinated Debt | Floating Interest Rate [Member]
Related Party Transaction [Line Items]
Available-for-sale Securities, Amortized Cost Basis	70,000				70,000			70,000	70,000
GSE Subordinated Debt | Floating Interest Rate [Member] | CoBank, ACB [Member]
Related Party Transaction [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 70,000							$ 70,000