QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ in Thousands	3 Months Ended								4 Months Ended	12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
QUARTERLY FINANCIAL DATA (UNAUDITED)
Operating revenues	$ 323,491	$ 384,791	$ 322,234	$ 323,462	$ 313,042	$ 388,102	$ 305,913	$ 328,391		$ 1,353,978	$ 1,335,448	$ 1,395,091
Operating margin	11,425	52,199	37,003	46,379	8,623	81,412	38,550	49,384		147,006	177,969	181,877
Net margins attributable to the Association	(6,061)	17,739	$ 2,537	$ 17,533	$ (21,299)	$ 48,302	$ 6,284	$ 20,126		31,748	53,413	64,236
Increase in operating revenues over comparable period in 2015	10,400
Recognition of deferred revenue										9,200		20,000
Increase in operating expenses over comparable period in 2015	7,600	25,900
Increase in purchased power expense over comparable period in 2015	16,700
Increase in depreciation, amortization and depletion expense over comparable period in 2015	4,500
Decrease in fuel expense over comparable period in 2015	$ 13,900
Increase in average cost per megawatt hour for purchased power over comparable period in 2015	3.50%
Increase in capital credits from cooperatives over comparable period in 2015	$ 11,400
Capital credit allocations										$ 20,349	$ 9,189	$ 8,684
Decrease in operating margins over comparable period in 2015		$ 29,200
Increase in net margins attributable to the Association over comparable period in 2015	15,200
Basin Electric Power Cooperative
QUARTERLY FINANCIAL DATA (UNAUDITED)
Capital credit allocations	14,400
Nucla Generating Station
QUARTERLY FINANCIAL DATA (UNAUDITED)
Increase in monthly depreciation expense									$ 700
Craig Generating Station Unit 1
QUARTERLY FINANCIAL DATA (UNAUDITED)
Increase in monthly depreciation expense									$ 200
Craig Generating Station Unit 1 | Nucla Generating Station
QUARTERLY FINANCIAL DATA (UNAUDITED)
Increase in depreciation, amortization and depletion expense over comparable period in 2015	3,600
Deferred revenue received in 2011
QUARTERLY FINANCIAL DATA (UNAUDITED)
Recognition of deferred revenue	$ 9,200