Commitments, Guarantees and Contingencies - Power Purchase Agreements (Details)	6 Months Ended		7 Months Ended
	Jun. 30, 2015 USD ($) MWh Years MW	Jun. 30, 2014 USD ($)	Dec. 31, 2014
Square Butte PPA [Member]
Power Purchase Agreements [Line Items]
PPA Counterparty Debt Outstanding | $	$ 389,400,000
Annual Debt Service | $	45,000,000
Power Purchased under Long-term Contracts | $	39,500,000	$ 29,800,000
Interest Expense | $	$ 2,500,000	$ 5,200,000
Square Butte PPA [Member] | Square Butte Coal-fired Unit [Member]
Power Purchase Agreements [Line Items]
Generating Unit Capacity (MW)	455
Output Entitlement	50.00%
Square Butte PPA [Member] | Minnkota Power Sales Agreement [Member] | Square Butte Coal-fired Unit [Member]
Power Purchase Agreements [Line Items]
Output Entitlement	28.00%		23.00%
Minnkota Power PPA [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	50
Oliver Wind I PPA [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	50
Fixed Capacity Charges | $	$ 0
Oliver Wind II PPA [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	48
Fixed Capacity Charges | $	$ 0
Manitoba Hydro PPA (expires May 2020) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	50
Manitoba Hydro PPA (expires April 2022) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MWh) | MWh	1,000,000
Manitoba Hydro PPA (expires 2035) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	250
Term of Contract (Years) | Years	15
Manitoba Hydro PPA (expires 2040) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	133
Term of Contract (Years) | Years	20
Great River Energy PPA (expires May 2020) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	50
Great River Energy Capacity Only PPA (expires May 2020) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	50