Commitments and Contingencies Environmental (Details) (USD $) In Millions, unless otherwise specified			9 Months Ended					9 Months Ended
	Sep. 30, 2013	Sep. 30, 2013 Colstrip Unit 4 [Member]	Sep. 30, 2013 Neal 4 Generating Facility [Member]	Jul. 01, 2018 Coyote Generating Facility [Member]	Sep. 30, 2013 Coyote Generating Facility [Member]	Sep. 30, 2013 Big Stone Generating Facility [Member]	Sep. 30, 2013 Manufactured Gas Plants [Member]	Sep. 30, 2013 Aberdeen South Dakota Site [Member] Manufactured Gas Plants [Member]
Environmental remediation obligation, minimum	$ 27.7
Environmental remediation obligation, maximum	35.7
Accrual for environmental loss contingencies	28.7						24.0	11.7
Environmental remediation obligation next 5 years								8.1
Jointly owned utility plant ownership percentage		30.00%	8.70%		10.00%	23.40%
NOx emissions per million Btu as calculated on a 30 day rolling average basis				0.5
Estimated capital expenditures for environmental obligations			270		9	405
Joint ownership share of capitalized project costs			$ 20.8			$ 27.6
Site contingency, time frame of disbursements			scheduled to commence in 2011 and be spread over the next three years.
Number of years for environmental remediation obligation to be incurred								5 years