Accumulated Other Comprehensive Income Amounts Reclassified (Details) - USD ($) $ in Thousands		3 Months Ended										12 Months Ended
		Dec. 31, 2017	[1]	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
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income (Loss) before income tax												$ 149,265	$ 150,686	$ 116,813
Income tax (expense) benefit												(51,004)	(53,918)	(34,895)
Net (Income) Loss		$ 6,500		$ 59,684	$ 36,066	$ (3,989)	$ 5,656	$ 74,636		$ 22,284	$ (5,808)	98,261	96,768	81,918
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Attributable to Parent
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	[3]											9,657	7,407	6,574
Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	[3]											(6,776)	(4,965)	(8,622)
Accumulated Defined Benefit Plans Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	[3]											2,881	2,442	(2,048)
Reclassification from AOCI, Current Period, Tax												(1,068)	(1,020)	810
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax												1,813	1,422	(1,238)
Accumulated Net Unrealized Investment Gain (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax												8,524	6,206	8,937
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax												(309)	(159)	(264)
Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax												10,028	7,469	7,435
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Net Unrealized Investment Gain (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net realized gain on sale of securities												10,626	7,640	11,114
Income (Loss) before income tax												10,626	7,640	11,114
Income tax (expense) benefit												(2,102)	(1,434)	(2,177)
Net (Income) Loss												8,524	6,206	8,937
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income (Loss) before income tax												(532)	(498)	(467)
Income tax (expense) benefit												223	339	203
Net (Income) Loss												(309)	(159)	(264)
Interest Rate Contract | Reclassification out of Accumulated Other Comprehensive Income | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest on long-term debt and revolving credit facility												$ (532)	$ (498)	$ (467)

[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.

[3] These items are included in the computation of net periodic benefit cost. See Part II, Item 8, Financial Statements and Supplementary Data, Note M for additional information.