Disaggregation of Revenues (Details) - USD ($) $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2018	[1],[2]	Sep. 30, 2018	[1],[2]	Jun. 30, 2018	[1],[2]	Mar. 31, 2018	[1],[2]	Dec. 31, 2017	[2],[3]	Sep. 30, 2017	[2]	Jun. 30, 2017	[2]	Mar. 31, 2017	[2]	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers																	$ 899,375
Other																	4,228
Total operating revenues	$ 190,823		$ 300,271		$ 236,796		$ 175,713		$ 196,149		$ 297,470		$ 251,843		$ 171,335		903,603	$ 916,797	$ 886,936
Retail
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers																	789,676
Wholesale
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers																	90,673
Wheeling (transmission)
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers																	$ 19,026

[1] Effective January 1, 2018, the Company implemented ASU 2016-01, Financial Instruments - Overall: Recognition and Measurement of Financial Assets and Liabilities. As required by the new standard, changes in the fair values of the Company's equity investments are recognized in earnings, whereas prior to 2018, such changes were recognized in accumulated other comprehensive income.

[2] Operating revenues are seasonal in nature, with the peak sales periods generally occurring during the summer months. Comparisons among quarters of a year may not represent overall trends and changes in operations.

[3] For financial reporting purposes, the Company deferred any recognition of the Company's request in its 2017 Texas Retail Rate Case until it received the 2017 PUCT Final Order on December 18, 2017. Accordingly, it reported in the fourth quarter of 2017 the cumulative effect of the 2017 PUCT Final Order, which related back to July 18, 2017. See Part II, Item 8, Financial Statements and Supplementary Data, Note D of Notes to Financial Statements.