COMMITMENTS AND CONTINGENCIES (Details) (USD $)	0 Months Ended	12 Months Ended	0 Months Ended	12 Months Ended
	Dec. 15, 2005	Dec. 31, 2014	Dec. 15, 2008	Dec. 31, 2013	Dec. 31, 2012
Commitments and Contingencies
Period of the average cost that is used as a basis for determining the maximum annual payments	20 years
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		$ 110,900,000
Expected cost of adding carbon injection system		112,000,000
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		165,000,000
Electric | High end of range | Power Plant Mercury And Air Toxics Rule
Commitments and Contingencies
Estimated project cost		175,000,000
Coal and coal transportation contracts
Commitments and Contingencies
January 1, 2015 through December 31, 2015		26,200,000
January 1, 2016 through December 31, 2017		29,500,000
January 1, 2018 through December 31, 2019		22,800,000
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		289,900,000
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		14,600,000
Purchased power | Elk River Windfarm
Commitments and Contingencies
Energy capacity (in megawatts)		150
Term of long-term contract agreement	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		16,900,000
Leases
Commitments and Contingencies
Gross amount of assets recorded under capital leases		5,300,000
Capital leases, 2015		553,000
Capital leases, 2016		549,000
Capital leases, 2017		547,000
Capital leases, 2018		547,000
Capital leases, 2019		546,000
Capital leases, thereafter		3,006,000
Capital leases, total minimum payments		5,748,000
Capital leases, amount representing interest		1,581,000
Capital leases, present value of net minimum lease payments		4,167,000
Operating leases, 2015		730,000
Operating leases, 2016		720,000
Operating leases, 2017		682,000
Operating leases, 2018		645,000
Operating leases, 2019		485,000
Operating leases, total minimum payments		3,262,000
Expenses incurred related to operating leases		800,000		800,000	900,000
Accumulated amount of amortization of capital leases		1,500,000		1,300,000
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
Firm Physical gas and transportation contracts
Commitments and Contingencies
January 1, 2015 through December 31, 2015		26,600,000
January 1, 2016 through December 31, 2017		42,900,000
January 1, 2018 through December 31, 2019		33,300,000
January 1, 2020 and beyond		$ 49,600,000