Condensed Statements of Patronage Capital and Membership Fees and Accumulated Other Comprehensive Margin (Deficit) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Increase (Decrease) in Members' Capital
Balance			$ 673,912	$ 634,307
Components of comprehensive margin:
Net margin	24,502	10,878	46,526	24,398
Unrealized (loss) gain on available-for-sale securities	(1,090)	120	(1,302)	828
Balance	719,136	659,533	719,136	659,533
Patronage Capital and Membership Fees
Increase (Decrease) in Members' Capital
Balance			673,009	633,689
Components of comprehensive margin:
Net margin			46,526	24,398
Balance	719,535	658,087	719,535	658,087
Accumulated Other Comprehensive Margin (Deficit)
Increase (Decrease) in Members' Capital
Balance			903	618
Components of comprehensive margin:
Unrealized (loss) gain on available-for-sale securities			(1,302)	828
Balance	$ (399)	$ 1,446	$ (399)	$ 1,446