Commitments and Contingencies (Detail) $ in Millions			3 Months Ended
	Dec. 15, 2008	Dec. 15, 2005	Mar. 31, 2016 USD ($) Office item MW
Electric | Power Plant Mercury And Air Toxics Rule
Commitments and Contingencies
Cost related to pre-engineering, site preparation activities and contract cost project to date			$ 163.4
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			$ 165.0
Electric | High end of range | Power Plant Mercury And Air Toxics Rule
Commitments and Contingencies
Estimated project cost			175.0
Firm Physical gas and transportation contracts
Commitments and Contingencies
April 1, 2016 through December 31, 2016			21.8
January 1, 2017 through December 31, 2018			37.4
January 1, 2019 through December 31, 2020			28.8
January 1, 2021 and beyond			45.7
Coal and coal transportation contracts
Commitments and Contingencies
April 1, 2016 through December 31, 2016			12.2
January 1, 2017 through December 31, 2018			26.9
January 1, 2019 through December 31, 2020			$ 10.5
Purchased power
Commitments and Contingencies
Number of purchased power agreement | item			3
Purchased power | Plum Point Energy Station
Commitments and Contingencies
Energy capacity (in megawatts) | MW			670
Amount of energy capacity owned (in megawatts) | MW			50
Amount of long-term contract obligation			$ 275.1
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			$ 0.0
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) | MW			150
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			$ 0.0
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 | item			2
Leases | Gas
Commitments and Contingencies
Number of office facilities under short-term operating leases | Office			1
Leases | Plum Point Energy Station
Commitments and Contingencies
Number of railcars under capital leases | item			108