Related Party Transactions (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended			3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Jun. 30, 2013 Subsidiary of Common Parent [Member]	Jun. 30, 2012 Subsidiary of Common Parent [Member]	Jun. 30, 2013 Subsidiary of Common Parent [Member]	Jun. 30, 2012 Subsidiary of Common Parent [Member]	Jun. 30, 2013 Energy sold to Cheyenne Light [Member]	Jun. 30, 2012 Energy sold to Cheyenne Light [Member]	Jun. 30, 2013 Energy sold to Cheyenne Light [Member]	Jun. 30, 2012 Energy sold to Cheyenne Light [Member]	Jun. 30, 2013 Rent from electric properties [Member]	Jun. 30, 2012 Rent from electric properties [Member]	Jun. 30, 2013 Rent from electric properties [Member]	Jun. 30, 2012 Rent from electric properties [Member]	Jun. 30, 2013 Coal, Purchased [Member]	Jun. 30, 2012 Coal, Purchased [Member]	Jun. 30, 2013 Coal, Purchased [Member]	Jun. 30, 2012 Coal, Purchased [Member]	Jun. 30, 2013 Allocated Costs From Related Parties [Member]	Jun. 30, 2012 Allocated Costs From Related Parties [Member]	Jun. 30, 2013 Allocated Costs From Related Parties [Member]	Jun. 30, 2012 Allocated Costs From Related Parties [Member]	Jun. 30, 2013 Purchase of Excess Energy, Cheyenne Light [Member]	Jun. 30, 2012 Purchase of Excess Energy, Cheyenne Light [Member]	Jun. 30, 2013 Purchase of Excess Energy, Cheyenne Light [Member]	Jun. 30, 2012 Purchase of Excess Energy, Cheyenne Light [Member]	Jun. 30, 2013 Happy Jack Wind Purchase Power Agreeement [Member] Renewable Wind Energy, Cheyenne Light [Member]	Jun. 30, 2012 Happy Jack Wind Purchase Power Agreeement [Member] Renewable Wind Energy, Cheyenne Light [Member]	Jun. 30, 2013 Happy Jack Wind Purchase Power Agreeement [Member] Renewable Wind Energy, Cheyenne Light [Member]	Jun. 30, 2012 Happy Jack Wind Purchase Power Agreeement [Member] Renewable Wind Energy, Cheyenne Light [Member]	Jun. 30, 2013 Silver Sage Wind Power Purchase Agreement [Member] Renewable Wind Energy, Cheyenne Light [Member]	Jun. 30, 2012 Silver Sage Wind Power Purchase Agreement [Member] Renewable Wind Energy, Cheyenne Light [Member]	Jun. 30, 2013 Silver Sage Wind Power Purchase Agreement [Member] Renewable Wind Energy, Cheyenne Light [Member]	Jun. 30, 2012 Silver Sage Wind Power Purchase Agreement [Member] Renewable Wind Energy, Cheyenne Light [Member]
Related Party Transaction [Line Items]
Non-cash Dividends Distributed to Parent	$ 0	$ 43,984
Non-cash Payment of Notes Receivable, Related Parties	0	43,984
Receivables - affiliates	4,955		5,027
Accounts payable - affiliates	15,739		21,896
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	1.00%
Related Party Transaction, Rate	1.75%
Money pool notes receivable, net	29,459		31,645
Net Interest Income (Expense), Related Party				138	168	288	468
Revenue from Related Parties								217	750	357	1,276	986	1,256	1,974	2,515
Costs and Expenses, Related Party																$ 4,741	$ 4,426	$ 9,265	$ 10,421	$ 7,328	$ 5,825	$ 15,054	$ 10,647	$ 969	$ 683	$ 1,915	$ 1,425	$ 415	$ 496	$ 1,065	$ 1,167	$ 711	$ 793	$ 1,796	$ 1,870