Regulatory Matters (Details) (USD $) In Millions, unless otherwise specified		3 Months Ended		6 Months Ended		12 Months Ended	9 Months Ended	3 Months Ended
	Sep. 30, 2013	Sep. 30, 2013 Demand-side management (DSM) lost revenues [Member]	Sep. 30, 2013 Montana Natural Gas Production Assets [Member] Revenue Subject to Refund [Member]	Jun. 30, 2012 Dave Gates Generating Station [Member]	Sep. 30, 2013 Dave Gates Generating Station [Member] Revenue Subject to Refund [Member]	Dec. 31, 2012 Deferred Revenue Related To Prior Year [Member] Demand-side management (DSM) lost revenues [Member]	Sep. 30, 2013 Deferred Revenue Related To Tracker Period 2012-2013 [Member] Demand-side management (DSM) lost revenues [Member]	Sep. 30, 2013 Deferred Revenue Related To Tracker Period 2013-2014 [Member] Demand-side management (DSM) lost revenues [Member]	Sep. 30, 2012 Bear Paw Basin [Member] Montana Natural Gas Production Assets [Member]
Customer refund liability, current			$ 3.2		$ 22.5
Deferred revenue recognized		4.6				2.3	7.8	1.9
Demand side management expected lost revenue recovery	7.8
Incremental costs disallowed				$ 1.4
Proportionate ownership share in gas gathering systems									75.00%