Regulatory Matters (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Public Utilities, General Disclosures [Line Items]
Federal statutory income tax rate	21.00%	35.00%
Deferred Income Tax Revenue [Member]
Public Utilities, General Disclosures [Line Items]
Deferred revenue	$ 7.3
Electric Supply Costs [Member]
Public Utilities, General Disclosures [Line Items]
Disallowed electric tracker costs				$ 12.4
Disallowed modeling costs				0.4
Dave Gates Generating Station [Member]
Public Utilities, General Disclosures [Line Items]
Deferred revenue, refund payments				$ 27.3
Montana Property Tax Tracker [Member]
Public Utilities, General Disclosures [Line Items]
Disallowed property tax costs			$ 1.7
Minimum [Member]
Public Utilities, General Disclosures [Line Items]
Expected annual revenue reduction	15.0
Potential customer refundable taxes	8.0
Maximum [Member]
Public Utilities, General Disclosures [Line Items]
Expected annual revenue reduction	20.0
Potential customer refundable taxes	$ 12.0