Commitments and Contingencies Environmental Liabilities (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended									12 Months Ended
	Dec. 31, 2011	Dec. 31, 2011 Manufactured Gas Plants [Member]	Dec. 31, 2011 Manufactured Gas Plants [Member] Combined Manufacturing Sites [Member]	Dec. 31, 2011 Manufactured Gas Plants [Member] Aberdeen South Dakota Site [Member]	Dec. 31, 2011 Clean Air Act Rules and Emission Control Expenditures [Member] Colstrip Unit 4 [Member] Jointly Owned Electricity Generation Plant [Member]	Dec. 31, 2011 Clean Air Act Rules and Emission Control Expenditures [Member] Big Stone Generating Facility [Member] Jointly Owned Electricity Generation Plant [Member]	Feb. 23, 2010 Clean Air Act Rules and Emission Control Expenditures [Member] Coyote Generating Facility [Member]	Dec. 31, 2011 Clean Air Act Rules and Emission Control Expenditures [Member] Coyote Generating Facility [Member] Jointly Owned Electricity Generation Plant [Member]	Dec. 31, 2011 Clean Air Act Rules and Emission Control Expenditures [Member] Neal 4 Generating Facility [Member] Jointly Owned Electricity Generation Plant [Member]	Dec. 31, 2011 Global Climate Change, GHG Emission [Member] tons
Environmental remediation obligation, minimum		$ 28.3
Environmental remediation obligation, maximum		37.5
Accrual for environmental loss contingencies		31.4	26.0	12.0
Environmental remediation obligation, to be incurred during next 5 years				9.2
Number of years for environmental remediation obligation to be incurred	5 years
Number of tons of greenhouse gases emitted per year resulting in permit required										100,000
Jointly owned utility plant, proportionate ownership share					30.00%	23.40%		10.00%	8.70%
Environmental obligation, estimated capital expenditure						$ 490		$ 6	$ 270
NOx emissions per million Btu							0.5