Consolidated Statements of Equity - USD ($) $ in Thousands	Total	Postretirement Medical Benefit Plans	Executive Benefit Restoration Plan	Patronage capital	Accumulated other comprehensive income (loss)	Accumulated other comprehensive income (loss) Postretirement Medical Benefit Plans	Accumulated other comprehensive income (loss) Executive Benefit Restoration Plan	Noncontrolling interest
Equity at beginning of period at Dec. 31, 2020				$ 978,519	$ (5,714)			$ 114,851
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net margins attributable to the Association	$ 26,346			26,346
Retirement of patronage capital				(10,000)
Unrealized loss on securities available for sale	(108)				(108)
Reclassification adjustment for actuarial (gain) loss		$ 78	$ 515			$ 78	$ 515
Reclassification adjustment for prior service cost (credit)		(2,139)	1,113			(2,139)	1,113
Curtailment and settlement			141				141
Unrecognized prior service credit (cost)		5,698	(1,050)			5,698	(1,050)
Unrecognized actuarial gain (loss)		784	(778)			784	(778)
Net comprehensive income attributable to noncontrolling interest	(6,942)							6,942
Equity distribution to noncontrolling interest								(2,693)
Equity at end of period at Dec. 31, 2021	1,112,505			994,865	(1,460)			119,100
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net margins attributable to the Association	0			0
Retirement of patronage capital				(10,000)
Unrealized loss on securities available for sale	(333)				(333)
Reclassification adjustment for actuarial (gain) loss		102	426			102	426
Reclassification adjustment for prior service cost (credit)		(1,636)	1,156			(1,636)	1,156
Curtailment and settlement			(187)				(187)
Unrecognized prior service credit (cost)		0	(308)			0	(308)
Unrecognized actuarial gain (loss)		32	1,740			32	1,740
Net comprehensive income attributable to noncontrolling interest	(8,400)							8,400
Equity distribution to noncontrolling interest								(1,320)
Equity at end of period at Dec. 31, 2022	1,110,577			984,865	(468)			126,180
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net margins attributable to the Association	10,087			10,087
Retirement of patronage capital				(10,371)
Unrealized loss on securities available for sale	127				127
Reclassification adjustment for actuarial (gain) loss		(84)	219			(84)	219
Reclassification adjustment for prior service cost (credit)		(1,637)	1,156			(1,637)	1,156
Curtailment and settlement			0				0
Unrecognized prior service credit (cost)		0	0			0	(909)
Unrecognized actuarial gain (loss)		$ 757	$ (909)			$ 757	$ 0
Net comprehensive income attributable to noncontrolling interest	(9,971)							9,971
Equity distribution to noncontrolling interest								(1,882)
Equity at end of period at Dec. 31, 2023	$ 1,118,011			$ 984,581	$ (839)			$ 134,269