Commitments, Guarantees and Contingencies - Power Purchase Agreements (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Power Purchase Agreements [Line Items]
Long-term Contract for Purchase of Electric Power, Number of Wind PPAs	2
Interest Expense	$ 43.6	$ 39.2	$ 33.8
Minnkota Power Sales Agreement [Member]
Power Purchase Agreements [Line Items]
Long-term Contract for Purchase of Electric Power, Date of Contract Expiration	2025
Long-term Contract for Purchase of Electric Power, Anticipated Date of New Transmission Line	2013
Oliver Wind I PPA [Member]
Power Purchase Agreements [Line Items]
Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased (MW)	50
Long-term Contract for Purchase of Electric Power, Start Date of Contract	2006
Oliver Wind II PPA [Member]
Power Purchase Agreements [Line Items]
Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased (MW)	48
Long-term Contract for Purchase of Electric Power, Start Date of Contract	2007
Oliver Wind I and II PPA [Member]
Power Purchase Agreements [Line Items]
Long-term Contract for Purchase of Electric Power, Term of Contract (Years)	25
fixed capacity charge	0
Manitoba Hydro PPA (expires April 2015) [Member]
Power Purchase Agreements [Line Items]
Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased (MW)	50
Long-term Contract for Purchase of Electric Power, Date of Contract Expiration	April 2015
Manitoba Hydro PPA (expires April 2022) [Member]
Power Purchase Agreements [Line Items]
Long-term Contract for Purchase of Electric Power, Start Date of Contract	May 2011
Long-term Contract for Purchase of Electric Power, Date of Contract Expiration	April 2022
Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased (MWh)	1,000,000
Long-term Contract for Purchase of Electric Power, Date of MPUC Approval	March 31, 2011
Manitoba Hydro PPA (expires 2035) [Member]
Power Purchase Agreements [Line Items]
Long Term Contract For Purchase Of Electric Power Date Agreement Was Executed	May 19, 2011
Long-term Contract for Purchase of Electric Power, Term of Contract (Years)	15
Long-term Contract for Purchase of Electric Power, Start Date of Contract	2020
Long-term Contract for Purchase of Electric Power, Generating Unit Capacity (MW)	250
Long-term Contract for Purchase of Electric Power, Date of MPUC Approval	January 26, 2012
Square Butte PPA [Member]
Power Purchase Agreements [Line Items]
Cost of Purchased Power	61.2	55.2	53.9
Long-term Contract for Purchase of Electric Power, Date of Contract Expiration	2026
Long-term Contract for Purchase of Electric Power, Generating Unit Capacity (MW)	455
Long-term Contract for Purchase of Electric Power, Amount of Long-term Debt or Lease Obligation Outstanding	451.4
Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased	50.00%
Long-term Contract for Purchase of Electric Power, Annual Minimum Debt Service Payment Required	44
Interest Expense	$ 11.1	$ 10.2	$ 11.0
DC Transmission Line from Square Butte [Member]
Power Purchase Agreements [Line Items]
Transmission Line Purchase Date	December 2009