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, 2016		Sep. 30, 2016	[2]	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
Selected Quarterly Financial Data
Operating revenues	$ 188,037	[1]	$ 323,225	[1]	$ 217,865	[1]	$ 157,809	[1]	$ 176,902	[1]	$ 289,713	[1]	$ 219,508	[1]	$ 163,746	[1]	$ 886,936	$ 849,869	$ 917,525
Operating income (loss)	20,470		129,857		44,697		(163)		8,312		88,047		41,872		7,960		194,861	146,191	151,163
Net income	$ 5,656		$ 74,636		$ 22,284		$ (5,808)		$ 648		$ 56,740		$ 21,072		$ 3,458		$ 96,768	$ 81,918	$ 91,428
Basic earnings per share:
Net income (loss)	$ 0.14		$ 1.84		$ 0.55		$ (0.14)		$ 0.02		$ 1.40		$ 0.52		$ 0.09		$ 2.390	$ 2.030	$ 2.270
Diluted earnings per share:
Net income (loss)	0.14		1.84		0.55		(0.14)		0.02		1.40		0.52		0.09		2.390	2.030	2.270
Dividends declared per share of common stock	$ 0.310		$ 0.310		$ 0.310		$ 0.295		$ 0.295		$ 0.295		$ 0.295		$ 0.280		$ 1.225	$ 1.165	$ 1.105

[1] 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.(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 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.

[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 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.