Common Stock (Details) - USD ($) $ / shares in Units, $ in Thousands					3 Months Ended									12 Months Ended
	Mar. 31, 2017	Mar. 17, 2017	Jan. 26, 2017	Dec. 30, 2016	Dec. 31, 2016	Sep. 30, 2016	[1]	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	May 29, 2014
Common Stock [Line Items]
Dividends, Common Stock, Cash				$ 12,600										$ 49,603	$ 47,059	$ 44,556
Common Stock, Dividends, Per Share, Declared					$ 0.310	$ 0.310		$ 0.310	$ 0.295	$ 0.295	$ 0.295	$ 0.295	$ 0.280	$ 1.225	$ 1.165	$ 1.105
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. 31, 2017
Dividends Payable, Date of Record		Mar. 17, 2017
Subsequent Event
Common Stock [Line Items]
Dividends Payable, Date Declared			Jan. 26, 2017
Common Stock, Dividends, Per Share, Declared			$ 0.31

[1] 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 PUCT Final Order on August 25, 2016. Accordingly, it reported in the third quarter of 2016 the cumulative effect of the PUCT Final Order which related back to January 12, 2016. See Part II, Item 8, Financial Statements and Supplementary Data, Note C.