Commitments, Guarantees and Contingencies - Power Purchase Agreements (Details) $ in Millions	6 Months Ended
	Jun. 30, 2016 USD ($) MW	Jun. 30, 2015 USD ($)
Square Butte [Member] | Square Butte PPA [Member]
Power Purchase Agreements [Line Items]
PPA Counterparty Debt Outstanding | $	$ 361.9
Cost of Power Purchased | $	37.7	$ 39.5
Pro Rata Share of Interest Expense | $	$ 4.8	$ 5.0
Square Butte [Member] | Square Butte PPA [Member] | Square Butte Coal-fired Unit [Member]
Power Purchase Agreements [Line Items]
Generating Capacity (MW)	455
Output Entitlement	50.00%
Minnkota Power [Member] | Square Butte Coal-fired Unit [Member] | Minnkota Power Sales Agreement [Member]
Power Purchase Agreements [Line Items]
Output Entitlement	28.00%	28.00%
Silver Bay Power [Member] | Silver Bay Power Sales Agreement through 2031 (Years 2016-2019) [Member] | Minimum [Member]
Power Purchase Agreements [Line Items]
Output Being Sold (MW)	50
Silver Bay Power [Member] | Silver Bay Power Sales Agreement through 2031 (Years 2020-2031) [Member]
Power Purchase Agreements [Line Items]
Output Being Sold (MW)	90
Silver Bay Power [Member] | Silver Bay Power Self-Generation [Member]
Power Purchase Agreements [Line Items]
Generating Capacity (MW)	90
Shell Energy [Member] | Shell Energy PPA [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	50