Commitments 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]
2016	$ 165,484
2017	133,534
2018	82,703
2019	49,196
2020	48,966
Thereafter	$ 130,745
Purchase Commitment [Member]
Long-term Purchase Commitment [Line Items]
Long Term Contract For Purchase of Fuel, Date of Contract Expiration	Dec. 31, 2017
Busch Ranch Wind Farm [Member] | Electric Utilities [Member]
Long-term Purchase Commitment [Line Items]
Jointly Owned Utility Plant, Proportionate Ownership Share Percentage	50.00%
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
Megawatts of Capacity Purchased | 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
Megawatts of Capacity Purchased | MW	50
Cost of Purchased Power	$ 1,213	1,227	1,384
Happy Jack Wind Purchase Power Agreeement [Member]
Long-term Purchase Commitment [Line Items]
Long-term Contract for Purchase of Electric Power, Date of Contract Expiration	Sep. 03, 2028
Megawatts of Capacity Purchased | MW	30
Happy Jack Wind Purchase Power Agreeement [Member] | Renewable Wind Energy, Cheyenne Light [Member]
Long-term Purchase Commitment [Line Items]
Cost of Purchased Power	$ 3,155	3,919	3,772
Silver Sage Wind Power Purchase Agreement [Member]
Long-term Purchase Commitment [Line Items]
Long-term Contract for Purchase of Electric Power, Date of Contract Expiration	Sep. 30, 2029
Megawatts of Capacity Purchased | MW	30
Silver Sage Wind Power Purchase Agreement [Member] | Renewable Wind Energy, Cheyenne Light [Member]
Long-term Purchase Commitment [Line Items]
Cost of Purchased Power	$ 4,107	4,798	4,809
Busch Ranch Wind Farm [Member]
Long-term Purchase Commitment [Line Items]
Long-term Contract for Purchase of Electric Power, Date of Contract Expiration	Oct. 16, 2037
Megawatts of Capacity Purchased | MW	14.5
Cost of Purchased Power	$ 1,734	1,998	1,856
Cargill Power Purchase Agreement [Member]
Long-term Purchase Commitment [Line Items]
Cost of Purchased Power	$ 16,112	$ 9,286	$ 12,291