Related Party Transactions (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) MW	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Related Party Transaction [Line Items]
Receivables - affiliates	$ 5,747	$ 5,350
Accounts payable - affiliates	$ 30,032	19,242
Utility Money Pool Interest Rate	1.45%
Notes Receivable (Payable), net - Utility Money Pool	$ 76,813	68,626
Net interest income (expense)	1,153	304	$ 505
Purchase of Excess Energy, Cheyenne Light [Member]
Related Party Transaction [Line Items]
Costs and Expenses	898	3,033	3,640
Parent [Member]
Related Party Transaction [Line Items]
Non-cash Dividend to Parent Company	$ 28,501	0	8,000
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	1.00%
Subsidiary of Common Parent [Member]
Related Party Transaction [Line Items]
Transfer from Investments	$ 28,501	0	8,000
Subsidiary of Common Parent [Member] | Purchase Of Natural Gas, Cheyenne Light [Member]
Related Party Transaction [Line Items]
Long-term Purchase Commitment, Period	40 years
Subsidiary of Common Parent [Member] | Coal, Purchased [Member]
Related Party Transaction [Line Items]
Costs and Expenses	$ 16,401	16,861	18,542
Subsidiary of Common Parent [Member] | Happy Jack Wind Purchase Power Agreeement [Member]
Related Party Transaction [Line Items]
Date of Contract Expiration	Sep. 03, 2028
Number of Megawatts Capacity Purchased | MW	15
Costs and Expenses	$ 1,578	1,959	1,886
Subsidiary of Common Parent [Member] | Silver Sage Wind Power Purchase Agreement [Member]
Related Party Transaction [Line Items]
Date of Contract Expiration	Sep. 30, 2029
Number of Megawatts Capacity Purchased | MW	20
Costs and Expenses	$ 2,739	3,200	3,207
Subsidiary of Common Parent [Member] | Allocated Costs From Related Parties [Member]
Related Party Transaction [Line Items]
Costs and Expenses	26,655	32,332	30,738
Subsidiary of Common Parent [Member] | Energy sold to Cheyenne Light [Member]
Related Party Transaction [Line Items]
Revenue	1,857	1,894	1,338
Subsidiary of Common Parent [Member] | Lease Agreements [Member]
Related Party Transaction [Line Items]
Revenue	$ 4,772	$ 4,102	$ 3,627