COMMITMENTS AND CONTINGENCIES (Detail) $ in Thousands			12 Months Ended
	Dec. 15, 2008	Dec. 15, 2005	Dec. 31, 2015 USD ($) item MW	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Commitments and Contingencies
Period of the average cost that is used as a basis for determining the maximum annual payments		20 years
Electric | Power Plant Mercury And Air Toxics Rule
Commitments and Contingencies
Construction costs year to date			$ 159,600
Cost related to pre-engineering, site preparation activities and contract cost year to date				$ 112,100
Electric | Low end of range | Power Plant Mercury And Air Toxics Rule
Commitments and Contingencies
Estimated project cost			165,000
Electric | High end of range | Power Plant Mercury And Air Toxics Rule
Commitments and Contingencies
Estimated project cost			175,000
Firm Physical gas and transportation contracts
Commitments and Contingencies
January 1, 2016 through December 31, 2016			26,700
January 1, 2017 through December 31, 2018			37,400
January 1, 2019 through December 31, 2020			28,800
January 1, 2021 and beyond			45,700
Coal and coal transportation contracts
Commitments and Contingencies
January 1, 2016 through December 31, 2016			18,000
January 1, 2017 through December 31, 2018			27,500
January 1, 2019 through December 31, 2020			$ 10,800
Purchased power | Plum Point Energy Station
Commitments and Contingencies
Energy capacity (in megawatts) | MW			670
Amount of energy capacity owned (in megawatts) | MW			50
Additional capacity under long-term contract agreement (in megawatts) | MW			50
Amount of long-term contract obligation			$ 277,600
Amount of energy capacity under option to purchase (in megawatts) | MW			50
Purchased power | Cloud County Windfarm
Commitments and Contingencies
Energy capacity (in megawatts) | MW			105
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
Purchased power | Elk River Windfarm
Commitments and Contingencies
Energy capacity (in megawatts) | MW			150
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
Leases
Commitments and Contingencies
Gross amount of assets recorded under capital leases			5,300
Capital leases, 2016			554
Capital leases, 2017			551
Capital leases, 2018			551
Capital leases, 2019			550
Capital leases, 2020			546
Capital leases, thereafter			2,460
Capital leases, total minimum payments			5,212
Capital leases, amount representing interest			1,322
Capital leases, present value of net minimum lease payments			3,890
Operating leases, 2016			734
Operating leases, 2017			689
Operating leases, 2018			648
Operating leases, 2019			484
Operating leases, total minimum payments			2,555
Expenses incurred related to operating leases			800	800	$ 800
Accumulated amount of amortization of capital leases			$ 1,900	$ 1,500
Leases | Electric
Commitments and Contingencies
Number of unit trains under short-term operating leases | item			2
Leases | Gas
Commitments and Contingencies
Number of office facilities under short-term operating leases | item			1
Leases | Plum Point Energy Station
Commitments and Contingencies
Number of railcars under capital leases | item			108