Commitments, Guarantees and Contingencies - Power Purchase Agreements (Details) $ in Millions	3 Months Ended
	Mar. 31, 2016 USD ($) MW	Mar. 31, 2015 USD ($)
Minnkota Power Sales Agreement [Member] | Square Butte Coal-fired Unit [Member]
Power Purchase Agreements [Line Items]
Output Entitlement	28.00%	28.00%
Square Butte PPA [Member]
Power Purchase Agreements [Line Items]
PPA Counterparty Debt Outstanding	$ 363.5
Cost of Power Purchased	18.5	$ 19.2
Pro Rata Share of Interest Expense	$ 2.4	$ 2.5
Square Butte PPA [Member] | Square Butte Coal-fired Unit [Member]
Power Purchase Agreements [Line Items]
Generating Capacity (MW) | MW	455
Output Entitlement	50.00%