COMMITMENTS AND CONTINGENCIES (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	0 Months Ended
	Mar. 31, 2015	Dec. 15, 2008	Dec. 15, 2005
Compliance Plan
Commitments and Contingencies
Amount of total power capacity of generating facility (in megawatts)	14
Electric | Power Plant Mercury And Air Toxics Rule
Commitments and Contingencies
Cost related to pre-engineering, site preparation activities and contract cost year to date	$ 117.7
Term of Capital Expenditure Plan	5 years
Electric | Low end of range | Power Plant Mercury And Air Toxics Rule
Commitments and Contingencies
Estimated project cost	165
Electric | High end of range | Power Plant Mercury And Air Toxics Rule
Commitments and Contingencies
Estimated project cost	175
Firm Physical gas and transportation contracts
Commitments and Contingencies
April 1, 2015 through December 31, 2015	17.7
January 1, 2016 through December 31, 2017	42.9
January 1, 2018 through December 31, 2019	33.6
January 1, 2020 and beyond	49.6
Coal and coal transportation contracts
Commitments and Contingencies
April 1, 2015 through December 31, 2015	17.8
January 1, 2016 through December 31, 2017	26.2
January 1, 2018 through December 31, 2019	21.9
Purchased power | Plum Point Energy Station
Commitments and Contingencies
Energy capacity (in megawatts)	670
Amount of energy capacity owned (in megawatts)	50
Term of long-term contract agreement	30 years
Amount of energy capacity under option to purchase (in megawatts)	50
Additional capacity under long-term contract agreement (in megawatts)	50
Amount of long-term contract obligation	285.0
Purchased power | Cloud County Windfarm
Commitments and Contingencies
Energy capacity (in megawatts)	105
Term of long-term contract agreement		20 years
Period of the average cost that is used as a basis for determining the maximum annual payments		20 years
Purchased power | Cloud County Windfarm | Low end of range
Commitments and Contingencies
Amount of annual payments contingent upon output of the facility	0
Purchased power | Cloud County Windfarm | High end of range
Commitments and Contingencies
Amount of annual payments contingent upon output of the facility	14.6
Purchased power | Elk River Windfarm
Commitments and Contingencies
Energy capacity (in megawatts)	150
Term of long-term contract agreement			20 years
Period of the average cost that is used as a basis for determining the maximum annual payments			20 years
Purchased power | Elk River Windfarm | Low end of range
Commitments and Contingencies
Amount of annual payments contingent upon output of the facility	0
Purchased power | Elk River Windfarm | High end of range
Commitments and Contingencies
Amount of annual payments contingent upon output of the facility	16.9
Leases
Commitments and Contingencies
Gross amount of assets recorded under capital leases	$ 5.3
Leases | Electric
Commitments and Contingencies
Number of unit trains under short-term operating leases	2
Leases | Gas
Commitments and Contingencies
Number of office facilities under short-term operating leases	1
Leases | Plum Point Energy Station
Commitments and Contingencies
Number of railcars under capital leases	108