COMMITMENTS AND CONTINGENCIES - Sales Commitments (Details)	12 Months Ended
Dec. 31, 2015 MW
Public Service Company of Colorado \| Supply Contracts
Sales commitments
Megawatts, summer season	125
Megawatts, winter season	175
Salt River Project Agricultural Improvement and Power District \| Long-term Supply Contracts
Sales commitments
Megawatts	100

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