Common Stock (Details) - USD ($) $ / shares in Units, $ in Thousands					3 Months Ended										12 Months Ended
	Mar. 30, 2018	Mar. 16, 2018	Feb. 01, 2018	Dec. 29, 2017	Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	[2]	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	May 29, 2014
Common Stock [Line Items]
Dividends, Common Stock, Cash				$ 13,600											$ 53,337	$ 49,603	$ 47,059
Common Stock, Dividends, Per Share, Declared					$ 0.335	[1]	$ 0.335	$ 0.335	$ 0.310	$ 0.310	$ 0.310		$ 0.310	$ 0.295	$ 1.315	$ 1.225	$ 1.165
Common Stock, Par or Stated Value Per Share					$ 1					$ 1					$ 1	$ 1
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized																		1,700,000.0
Restricted Stock | Minimum
Common Stock [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Requisite Service Period															1 year
Restricted Stock | Maximum
Common Stock [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Requisite Service Period															3 years
Scenario, Forecast
Common Stock [Line Items]
Dividends Payable, Date to be Paid	Mar. 30, 2018
Dividends Payable, Date of Record		Mar. 16, 2018
Subsequent Event
Common Stock [Line Items]
Dividends Payable, Date Declared			Feb. 01, 2018
Common Stock, Dividends, Per Share, Declared			$ 0.335

[1] 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 C.

[2] For financial reporting purposes, the Company deferred any recognition of the Company's request in its 2015 Texas Retail Rate Case until it received the 2016 PUCT Final Order on August 25, 2016. Accordingly, it reported in the third quarter of 2016 the cumulative effect of the 2016 PUCT Final Order which related back to January 12, 2016. See Part II, Item 8, Financial Statements and Supplementary Data, Note C.