Regulatory Matters (Details) (USD $) In Millions, unless otherwise specified		24 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2014 Montana Natural Gas Production Assets [Member] Revenue Subject to Refund [Member]	Sep. 30, 2014 Dave Gates Generating Station [Member] Revenue Subject to Refund [Member]	Sep. 30, 2014 Regulatory Reviews of Filings [Member]
Customer refund liability, current			$ 27.3
CU4 incremental market purchases identified for further review				11
Revenue recognized, subject to refund		22.8
Demand side management expected lost revenue recovery	$ 7.1