JOINTLY OWNED UTILITY FACILITIES (Details) $ in Millions	Dec. 31, 2017 USD ($) MW
Weston 4
Share of significant jointly owned electric generating facilities
Ownership (as a percent)	70.00%
Our share of rated capacity (in megawatts) | MW	383.9
Utility plant	$ 600.5
Accumulated depreciation	(189.2)
Construction work in progress	$ 5.3
Columbia Energy Center Units 1 and 2
Share of significant jointly owned electric generating facilities
Ownership (as a percent)	29.50%
Our share of rated capacity (in megawatts) | MW	319.7
Utility plant	$ 412.7
Accumulated depreciation	(127.3)
Construction work in progress	$ 27.6
Edgewater Unit 4
Share of significant jointly owned electric generating facilities
Ownership (as a percent)	31.80%
Our share of rated capacity (in megawatts) | MW	98.0
Utility plant	$ 45.9
Accumulated depreciation	(32.9)
Construction work in progress	$ 0.0
WPSC | Columbia Energy Center Units 1 and 2
Share of significant jointly owned electric generating facilities
Future Ownership Interest of Columbia	27.50%