ORGANIZATION - Customer (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) employee customer agreement	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Electricity sales
Number of members exceeding 10% of revenues	43
Member electric sales | $	$ 1,199,940	$ 1,134,781	$ 1,125,699
Number of employees | employee	1,546
Total Operating Revenues | Customer Concentration | United Power, Inc.
Electricity sales
Number of members exceeding 10% of revenues	1
Percentage of benchmark	12.10%
Member Revenues | Customer Concentration | United Power, Inc.
Electricity sales
Number of members exceeding 10% of revenues	1
Member electric sales | $	$ 168,800
Percentage of benchmark	14.10%
Workforce Subject to Collective Bargaining Arrangements | Unionized Employees Concentration Risk
Electricity sales
Number of employees | employee	319
Workforce Subject to Collective Bargaining Arrangements Expiring within One Year | Unionized Employees Concentration Risk
Electricity sales
Number of collective bargaining agreements expiring within one year | agreement	0