Consolidated Statements of Patronage Capital and Membership Fees - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Components of comprehensive margin:
Net margin	$ 9,383		$ 17,285		$ 32,979	$ 44,684
Patronage Capital and Membership Fees
Increase (Decrease) in Members' Capital
Beginning balance	985,882	$ 962,286	938,486	$ 911,087	962,286	911,087
Components of comprehensive margin:
Net margin	9,383	23,596	17,285	27,399
Ending balance	$ 995,265	$ 985,882	$ 955,771	$ 938,486	$ 995,265	$ 955,771