Commitment and Contingencies (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) MW	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Contractual Obligation, Fiscal Year Maturity [Abstract]
2015	$ 12,827
2016	12,824
2017	6,513
2018	6,408
2019	5,880
Thereafter	$ 17,641
M D U, Montana Dakota Utilities [Member] | Wygen I I I Generating Facility [Member]
Sales Capacity Commitments [Abstract]
Number of MW Sold Under Long-Term Contract | MW	25
City Of Gillette [Member] | Wygen I I I Generating Facility [Member]
Sales Capacity Commitments [Abstract]
Number of MW Sold Under Long-Term Contract | MW	23
Purchase Power Contract, MEAN, 10 M W [Member]
Sales Capacity Commitments [Abstract]
Long-term Contract To Sell Electric Power, Date of Contract Expiration	May 31, 2023
Purchase Power Contract, MEAN, 10 M W [Member] | Wygen I I I Generating Facility [Member]
Sales Capacity Commitments [Abstract]
Number of MW Sold Under Long-Term Contract | MW	10
Purchase Power Contract, MEAN, 10 M W [Member] | Neil Simpson I I [Member]
Sales Capacity Commitments [Abstract]
Number of MW Sold Under Long-Term Contract | MW	10
Thunder Creek - Gas Transport Capacity [Member]
Long-term Purchase Commitment [Line Items]
Long Term Contract For Purchase of Fuel, Date of Contract Expiration	Oct. 31, 2019
Gas Gathering, Transportation, Marketing and Processing Costs	$ 633	$ 633	$ 633
PacifiCorp Purchase Power Agreement [Member]
Long-term Purchase Commitment [Line Items]
Long-term Contract for Purchase of Electric Power, Date of Contract Expiration	Dec. 31, 2023
Number of M W Capacity Purchased Under Long-term Contract | MW	50
Cost of Purchased Power	$ 13,990	13,943	13,026
PacifiCorp Transmission [Member]
Long-term Purchase Commitment [Line Items]
Long-term Contract for Purchase of Electric Power, Date of Contract Expiration	Dec. 31, 2023
Number of M W Capacity Purchased Under Long-term Contract | MW	50
Cost of Purchased Power	$ 1,213	$ 1,227	$ 1,384
Maximum [Member] | M D U, Montana Dakota Utilities [Member]
Sales Capacity Commitments [Abstract]
Number of MW Sold Under Long-Term Contract | MW	50