Commitments and Contingencies Environmental Liabilities (Details) (USD $) In Millions, unless otherwise specified			12 Months Ended
	Dec. 31, 2012 Environmental remediation obligations [Member]	Dec. 31, 2012 Manufactured Gas Plants [Member] Combined Manufacturing Sites [Member]	Dec. 31, 2012 Manufactured Gas Plants [Member] Aberdeen South Dakota Site [Member]	Dec. 31, 2012 Clean Air Act Rules and Emission Control Expenditures [Member] Colstrip Unit 4 [Member] Jointly Owned Electricity Generation Plant [Member]	Dec. 31, 2012 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, 2012 Clean Air Act Rules and Emission Control Expenditures [Member] Coyote Generating Facility [Member] Jointly Owned Electricity Generation Plant [Member]	Dec. 31, 2012 Clean Air Act Rules and Emission Control Expenditures [Member] Neal 4 Generating Facility [Member] Jointly Owned Electricity Generation Plant [Member]
Environmental remediation obligation, minimum	$ 28.3
Environmental remediation obligation, maximum	36.4
Accrual for environmental loss contingencies	31.5	26.2	12.4
Environmental remediation obligation, to be incurred during next 5 years			8.8
Number of years for environmental remediation obligation to be incurred			5 years
Environmental obligation, estimated capital expenditures					$ 490		$ 6	$ 270
Jointly owned utility plant, proportionate ownership share				30.00%	23.40%		10.00%	8.70%
NOx emissions per million Btu						0.5