Condensed Statements of Patronage Capital and Membership Fees and Accumulated Other Comprehensive Margin (Deficit) (Equity) (USD $) In Thousands, unless otherwise specified	Total	Patronage Capital and Membership Fees	Accumulated Other Comprehensive Margin (Deficit)
Balance at Dec. 31, 2010	$ 595,483	$ 595,952	$ (469)
Components of comprehensive margin:
Net margin	28,828	28,828
Unrealized gain on available-for-sale securities	592		592
Balance at Jun. 30, 2011	624,903	624,780	123
Balance at Dec. 31, 2011	634,307	633,689	618
Components of comprehensive margin:
Net margin	24,398	24,398
Unrealized gain on available-for-sale securities	828		828
Balance at Jun. 30, 2012	$ 659,533	$ 658,087	$ 1,446