PRESENTATION OF FINANCIAL INFORMATION - Depreciation Rates (Details) - Forecast $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($)
Transmission and General Plant \| Adopted Depreciation Rates
Depreciation rates
Expected increase (decrease) in annual depreciation expense associated with revised depreciation rates	$ (21.0)
Generation plant
Depreciation rates
Expected increase (decrease) in annual depreciation expense associated with revised depreciation rates	$ 2.0

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