Common Stock (Basic and Diluted Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended									12 Months Ended
	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
Class of Stock Disclosures [Abstract]
Basic number of common shares outstanding										40,350,688	40,274,986	40,190,991
Dilutive effect of unvested performance awards										57,345	33,576	20,726
Diluted number of common shares outstanding										40,408,033	40,308,562	40,211,717
Net income	$ 5,656	$ 74,636		$ 22,284	$ (5,808)	$ 648	$ 56,740	$ 21,072	$ 3,458	$ 96,768	$ 81,918	$ 91,428
Income allocated to participating restricted stock										(321)	(243)	(301)
Net income available to common shareholders basic										96,447	81,675	91,127
Income reallocated to participating restricted stock										(321)	(243)	(301)
Net income available to common shareholders diluted										$ 96,447	$ 81,675	$ 91,127
Earnings Per Share, Basic, Distributed										$ 1.225	$ 1.165	$ 1.105
Earnings Per Share, Basic, Undistributed										1.165	0.865	1.165
Basic net income per common share	$ 0.14	$ 1.84		$ 0.55	$ (0.14)	$ 0.02	$ 1.40	$ 0.52	$ 0.09	2.390	2.030	2.270
Earnings Per Share, Diluted, Distributed										1.225	1.165	1.105
Earnings Per Share, Diluted, Undistributed										1.165	0.865	1.165
Diluted net income per common share	$ 0.14	$ 1.84		$ 0.55	$ (0.14)	$ 0.02	$ 1.40	$ 0.52	$ 0.09	$ 2.390	$ 2.030	$ 2.270

[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.