Consolidated Statements of Patronage Capital and Membership Fees and Accumulated Other Comprehensive Deficit - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Increase (Decrease) in Members' Capital
Balance			$ 911,087	$ 859,440
Components of comprehensive margin:
Net margin	$ 17,285	$ 15,676	44,684	37,130
Unrealized gain (loss) on available-for-sale securities		1		(38)
Balance	955,771	896,532	955,771	896,532
Patronage Capital and Membership Fees
Increase (Decrease) in Members' Capital
Balance			911,087	859,810
Components of comprehensive margin:
Net margin			44,684	37,130
Balance	$ 955,771	896,940	$ 955,771	896,940
Accumulated Other Comprehensive Deficit
Increase (Decrease) in Members' Capital
Balance				(370)
Components of comprehensive margin:
Unrealized gain (loss) on available-for-sale securities				(38)
Balance		$ (408)		$ (408)