Condensed Statements of Patronage Capital and Membership Fees and Accumulated Other Comprehensive Margin (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Increase (Decrease) in Members' Capital
Balance	$ 673,912	$ 634,307
Components of comprehensive margin:
Net margin	22,024	13,520
Unrealized (loss) gain on available-for-sale securities	(212)	709
Balance	695,724	648,536
Patronage Capital and Membership Fees
Increase (Decrease) in Members' Capital
Balance	673,009	633,689
Components of comprehensive margin:
Net margin	22,024	13,520
Balance	695,033	647,209
Accumulated Other Comprehensive Margin
Increase (Decrease) in Members' Capital
Balance	903	618
Components of comprehensive margin:
Unrealized (loss) gain on available-for-sale securities	(212)	709
Balance	$ 691	$ 1,327