LEASES - Lessor (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) facility MW	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Operating leases - LESSOR
Generating units, accumulated depreciation	$ 2,409,020	$ 2,361,555
Gas tolling arrangement
Operating leases - LESSOR
Lease revenues	11,600	17,100	$ 30,100
Generation plant	111,000	114,000
Generating units, accumulated depreciation	$ 63,000	$ 63,000
Gas tolling arrangement | J. M. Shafer Generating Station
Operating leases - LESSOR
Megawatt usage | MW	272
Gas tolling arrangement | J. M. Shafer Generating Station | Third Party
Operating leases - LESSOR
Megawatt usage | MW	122
Gas tolling arrangement | Knutson Generating Station | Third Party
Operating leases - LESSOR
Number of generating station units | facility	2
Gas tolling arrangement | Limon Generating Station
Operating leases - LESSOR
Number of generating station units | facility	2
Gas tolling arrangement | Limon Generating Station | Third Party
Operating leases - LESSOR
Number of generating station units | facility	1