Commitments, Guarantees and Contingencies - Power Purchase Agreements (Details)	7 Months Ended	9 Months Ended
	Dec. 31, 2014	Sep. 30, 2015 USD ($) MWh Years MW	Sep. 30, 2014 USD ($)
Square Butte PPA [Member]
Power Purchase Agreements [Line Items]
PPA Counterparty Debt Outstanding | $		$ 377,900,000
Annual Debt Service | $		45,000,000
Power Purchased under Long-term Contracts | $		57,600,000	$ 51,800,000
Interest Expense | $		$ 7,600,000	$ 7,900,000
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	23.00%	28.00%
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 and Capacity PPA (expires May 2020) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)		50
Great River Capacity Only PPA (expires May 2020) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)		50
TransAlta Off-Peak Hours PPA (expires 2019) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)		50
TransAlta On-Peak Hours PPA (expire 2019) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)		100