Selected Quarterly Financial Data (Unaudited) Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2018	[1]	Sep. 30, 2018	[1]	Jun. 30, 2018	[1]	Mar. 31, 2018	[1]	Dec. 31, 2017	[3]	Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Selected Quarterly Financial Data
Operating revenues	$ 190,823	[2]	$ 300,271	[2]	$ 236,796	[2]	$ 175,713	[2]	$ 196,149	[2]	$ 297,470	[2]	$ 251,843	[2]	$ 171,335	[2]	$ 903,603	$ 916,797	$ 886,936
Operating income	15,113	[4]	99,933	[4]	53,139	[4]	4,044	[4]	18,250	[4]	103,688	[4]	63,916	[4]	4,205	[4]	172,229	190,059	187,911
Net income	$ (15,285)		$ 73,271		$ 33,295		$ (6,966)		$ 6,500		$ 59,684		$ 36,066		$ (3,989)		$ 84,315	$ 98,261	$ 96,768
Basic earnings per share:
Net income (loss)	$ (0.38)		$ 1.80		$ 0.82		$ (0.17)		$ 0.16		$ 1.47		$ 0.89		$ (0.10)		$ 2.070	$ 2.420	$ 2.390
Diluted earnings per share:
Net income (loss)	(0.38)		1.79		0.82		(0.17)		0.16		1.47		0.89		(0.10)		2.070	2.420	2.390
Dividends declared per share of common stock	$ 0.360		$ 0.360		$ 0.360		$ 0.335		$ 0.335		$ 0.335		$ 0.335		$ 0.310		$ 1.415	$ 1.315	$ 1.225

[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.

[4] The Company implemented ASU 2017-07, Compensation - Retirement Benefits, in the first quarter of 2018, and as required by the standard, reclassified certain amounts in the financial statements for 2017. See Part II, Item 8, Financial Statements and Supplementary Data, Notes B and N of Notes to Financial Statements.