Commitments, Guarantees and Contingencies - Power Purchase Agreements (Details) (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Square Butte PPA [Member]
Power Purchase Agreements [Line Items]
PPA Counterparty Debt Outstanding	$ 423,400,000
Annual Debt Service	44,000,000
Power Purchased Under Long-term Contracts	16,800,000	16,300,000
Interest Expense	2,500,000	2,600,000
Square Butte PPA [Member] | Square Butte Coal-fired Unit [Member]
Power Purchase Agreements [Line Items]
Generating Capacity (MW)	455
Output Entitlement	50.00%
Minnkota 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
Term of Contract (Years)	25
Fixed Capacity Charges	0
Oliver Wind II PPA [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	48
Term of Contract (Years)	25
Fixed Capacity Charges	$ 0
Manitoba Hydro PPA (expires April 2015) [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)	1,000,000
Manitoba Hydro PPA (expires 2035) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	250
Term of Contract (Years)	15
Interstate Power and Light Company PPA (expires April 2019) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MWh)	219,000
Interstate Power and Light Company (expires June 2032) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MWh)	26,000