Regulatory Matters (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	41 Months Ended
	Jun. 30, 2016	Sep. 30, 2016	Nov. 30, 2015	Mar. 31, 2016	Dec. 31, 2015
Public Utilities, General Disclosures [Line Items]
Property, plant and equipment, net		$ 4,161,993			$ 4,059,499
Montana Natural Gas Rate Filing [Member]
Public Utilities, General Disclosures [Line Items]
Requested rate increase		10,900
Requested rate increase for delivery service		7,400
Requested rate increase for production		$ 3,500
Requested return on equity, percentage		10.35%
Public utilities rate base		$ 432,100
Requested debt capital structure, percentage		53.00%
Requested equity capital structure, percentage		47.00%
Interim rate increase		$ 5,600
Hydro Transaction [Member] | Revenue Subject to Refund [Member]
Public Utilities, General Disclosures [Line Items]
Deferred revenue, current		2,600
Demand side management [Member]
Public Utilities, General Disclosures [Line Items]
Demand side management revenue recognized annually			$ 7,100
Deferred revenue recognized	$ 14,200
Deferred revenue					$ 14,200
Dave Gates Generating Station [Member]
Public Utilities, General Disclosures [Line Items]
Deferred revenue, refund payments	27,300
Property, plant and equipment, net		160,000
Disallowed Expenses [Member]
Public Utilities, General Disclosures [Line Items]
Disallowed replacement power costs				$ 8,200
Disallowed costs		12,400		10,300
Disallowed interest costs		$ 2,900
Disallowed modeling costs				$ 2,100
Disallowed modeling cost reduction	$ 800