Commitments and Contingencies Environmental (Details) (USD $) In Millions, unless otherwise specified							9 Months Ended
	Sep. 30, 2014	Sep. 30, 2014 Colstrip Unit 4 [Member]	Sep. 30, 2014 Neal 4 Generating Facility [Member]	Sep. 30, 2014 Coyote Generating Facility [Member]	Sep. 30, 2014 Big Stone Generating Facility [Member]	Sep. 30, 2014 Manufactured Gas Plants [Member]	Sep. 30, 2014 Aberdeen South Dakota Site [Member] Manufactured Gas Plants [Member]	Jul. 01, 2018 Scenario, Forecast [Member] Coyote Generating Facility [Member]
Environmental remediation obligation, minimum	$ 27.3
Environmental remediation obligation, maximum	35.0
Accrual for environmental loss contingencies	28.7					22.2	11.4
Environmental remediation obligation next 5 years							8.3
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				9.0	384.0
Joint ownership share of capitalized project costs					$ 64.4
Number of years for environmental remediation obligation to be incurred							5 years