ORGANIZATION - Electric Sales (Details) MWh in Millions, $ in Billions	12 Months Ended
	Dec. 31, 2017 USD ($) MWh customer	Dec. 31, 2016 USD ($) MWh customer	Dec. 31, 2015 USD ($) MWh
Electricity sales
Number of Members	43
Total megawatt-hours sold | MWh	18.0	17.9	17.8
Total electric sales | $	$ 1.3	$ 1.3	$ 1.2
Energy resources, percentage supplied by generation	61.40%	60.80%	63.30%
Members
Electricity sales
Number of Members	43	43
Total Megawatt-Hours | Customer Concentration | Members
Electricity sales
Percentage of benchmark	88.30%	88.00%	88.60%
Electric sales provided by generation | Customer Concentration | Members
Electricity sales
Percentage of benchmark	92.30%	90.50%	90.30%