ORGANIZATION - Long Term Contracts (Details)	12 Months Ended
	Dec. 31, 2017 customer MW	Dec. 31, 2016 customer
Member contracts
Number of Members	43
Members
Member contracts
Number of Members	43	43
Minimum percent of purchase and receive obligation (as a percent)	95.00%
Long-term Supply Contracts | Members
Member contracts
Minimum termination notice period	2 years
Contracts expiring in 2050 | Members
Member contracts
Number of Members	42
Percentage of revenue	96.80%
Long-term Supply Contract, Five Percent Program | Members
Member contracts
Maximum percent of transmission or generation providable by member	5.00%
Number of Members exercising Election option	22
Unit capacity (in megawatts) | MW	143
Capacity in operation (in megawatts) | MW	98