Commitments and Contingencies Environmental (Details) (USD $) In Millions, unless otherwise specified							9 Months Ended
	Sep. 30, 2012	Sep. 30, 2012 Manufactured Gas Plants [Member]	Sep. 30, 2012 Manufactured Gas Plants [Member] Aberdeen South Dakota Site [Member]	Sep. 30, 2012 Clean Air Act Rules and Associated Emission Control Equipment Expenditures [Member] Big Stone Generating Facility [Member] Jointly Owned Electricity Generation Plant [Member]	Feb. 23, 2010 Clean Air Act Rules and Associated Emission Control Equipment Expenditures [Member] Coyote Generating Facility [Member]	Sep. 30, 2012 Clean Air Act Rules and Associated Emission Control Equipment Expenditures [Member] Coyote Generating Facility [Member] Jointly Owned Electricity Generation Plant [Member]	Sep. 30, 2012 Clean Air Act Rules and Associated Emission Control Equipment Expenditures [Member] Neal 4 Generating Facility [Member] Jointly Owned Electricity Generation Plant [Member]	Sep. 30, 2012 Clean Air Act Rules and Associated Emission Control Equipment Expenditures [Member] Colstrip Unit 4 [Member] Jointly Owned Electricity Generation Plant [Member]
Environmental remediation obligation, minimum	$ 28.3
Environmental remediation obligation, maximum	37.5
Accrual for environmental loss contingencies	31.0	25.6	11.6
Environmental remediation obligation next 5 years			9.1
Jointly owned utility plant ownership percentage				23.40%		10.00%	8.70%	30.00%
NOx emissions per million Btu as calculated on a 12-month rolling basis					0.5
Estimated capital expenditures for environmental obligations				$ 490.0		$ 6.0	$ 270.0
Site contingency, time frame of disbursements							scheduled to commence in 2011 and be spread over the next three years.