Income Taxes (Details)	1 Months Ended	3 Months Ended	12 Months Ended
	Feb. 29, 2016	Sep. 30, 2015	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2012	Dec. 31, 2016 USD ($) $ / kWh	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Consolidated Appropriations Act of 2016 [Abstract]
Excise Tax Delay			2 years
Unrecognized Tax Benefits, Interest on Income Taxes Accrued							$ (900,000)	$ 0	$ (100,000)	$ 0
Interest Income (Expense) related to unrecognized tax benefits			$ (900,000)	$ 100,000	$ (100,000)
Unrecognized Tax Benefits, Income Tax Penalties Accrued							0	0	0
Tax Increase Prevention Act of 2014 [Abstract]
Number of years bonus depreciation was extended					1 year
Unrecognized Tax Benefits [Abstract]
Unrecognized tax benefit — Permanent tax positions							4,500,000	2,600,000
Unrecognized tax benefit — Temporary tax positions							24,200,000	22,100,000
Total unrecognized tax benefit			24,700,000	13,200,000	$ 4,100,000		28,700,000	24,700,000	$ 13,200,000	$ 4,100,000
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at Jan. 1			24,700,000	13,200,000	4,100,000
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions			1,400,000	4,200,000	8,600,000
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions			0	(600,000)	0
Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions			3,900,000	9,000,000	2,300,000
Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions			(1,300,000)	(1,100,000)	(300,000)
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities			0	0	(200,000)
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations			0	0	(1,300,000)
Balance at Dec. 31			$ 28,700,000	$ 24,700,000	$ 13,200,000
Tax Benefits Associated With NOL And Tax Credit Carryforwards [Abstract]
NOL and tax credit carryforwards							(5,900,000)	(5,000,000)
Decrease in Unrecognized Tax Benefits is Reasonably Possible							10,000,000
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent			35.00%	35.00%	35.00%
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent			1.50%	2.60%	3.40%
Effective Income Tax Rate Reconciliation Change In Unrecognized Tax Benefits, Percent			0.80%	0.50%	0.20%
Effective Income Tax Rate Reconciliation Regulatory Differences Utility Plant Items, Percent			(1.00%)	(0.80%)	(1.60%)
Effective Income Tax Rate Reconciliation, Tax Credit, Percent			(0.50%)	(0.30%)	(0.40%)
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent			(0.70%)	0.10%	0.10%
Effective Income Tax Rate Reconciliation, Percent			35.10%	37.10%	36.70%
Components of Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)			$ (40,853,000)	$ (1,327,000)	$ (57,201,000)
Current State and Local Tax Expense (Benefit)			(2,929,000)	2,448,000	2,512,000
Current Change In Unrecognized Tax Expense (Benefit)			3,126,000	11,281,000	6,715,000
Deferred Federal Income Tax Expense (Benefit)			116,404,000	67,640,000	121,882,000
Deferred State and Local Income Tax Expense (Benefit)			7,757,000	5,399,000	8,025,000
Deferred Change In Unrecognized Tax Expense (Benefit)			(1,178,000)	(10,203,000)	(6,390,000)
Deferred investment tax credits			(213,000)	(213,000)	(341,000)
Income Tax Expense (Benefit)			82,114,000	75,025,000	75,202,000
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred tax expense (benefit) excluding selected items			128,393,000	63,453,000	124,875,000
Amortization and adjustments to deferred income taxes on income tax regulatory assets and liabilities			(5,416,000)	(780,000)	(1,262,000)
Other Comprehensive Income (Loss), Tax			6,000	163,000	(96,000)
Deferred Income Tax Expense (Benefit)			122,983,000	$ 62,836,000	$ 123,517,000
Deferred Tax Liabilities, Gross [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment							1,034,675,000	945,142,000
Deferred Tax Liabilities, Compensation and Benefits, Employee Benefits							42,239,000	50,097,000
Deferred Tax Liabilities, Other							35,975,000	18,260,000
Deferred Tax Liabilities, Gross							1,112,889,000	1,013,499,000
Deferred Tax Assets, Gross [Abstract]
Deferred Tax Assets, Operating Loss Carryforwards							100,179,000	112,060,000
Deferred Tax Assets Unbilled Revenue Fuel Costs							10,226,000	23,127,000
Deferred Tax Assets Regulatory Liabilities							3,380,000	10,480,000
Deferred Tax Assets, Other							9,967,000	7,088,000
Deferred Tax Assets, Net of Valuation Allowance							123,752,000	152,755,000
Deferred Tax Liabilities, Net							989,137,000	860,744,000
Internal Revenue Service (IRS)
Tax Audits [Abstract]
Year(s) under examination		2012 and 2013				2010 and 2011
Year of carryback claim under examination						2009
Potential Tax Adjustments			$ 14,000,000
Earliest year subject to examination			2009
Operating Loss Carryforwards							275,000,000	306,000,000
Tax Credit Carryforward, Amount							4,000,000	3,000,000
Carryforward expiration date range, low			2021
Carryforward expiration date range, high			2036
State and Local Jurisdiction
Tax Audits [Abstract]
Earliest year subject to examination			2009
Operating Loss Carryforwards							60,000,000	79,000,000
Valuation Allowance for Tax Credit Carryforward Net of Federal Benefit							$ 0	$ (11,000,000)
Carryforward expiration date range, low			2017
Carryforward expiration date range, high			2035
TEXAS
Tax Audits [Abstract]
Year(s) under examination	2009 and 2010
Consolidated Appropriations Act of 2016; 2015, 2016, 2017 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Bonus depreciation rate, Percent							50.00%
Consolidated Appropriations Act of 2016; 2018 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Bonus depreciation rate, Percent							40.00%
Production Tax Credit Rate, Percent							60.00%
Consolidated Appropriations Act of 2016; 2019 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Bonus depreciation rate, Percent							30.00%
Production Tax Credit Rate, Percent							40.00%
Investment Tax Credit Rate, Percent							30.00%
Consolidated Appropriations Act of 2016; 2016 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Production Tax Credit Rate, Percent							100.00%
Production Tax Credit per KWh | $ / kWh							0.023
Consolidated Appropriations Act of 2016; 2017 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Production Tax Credit Rate, Percent							80.00%
Consolidated Appropriations Act of 2016; 2020 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Investment Tax Credit Rate, Percent							26.00%
Consolidated Appropriations Act of 2016; 2021 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Investment Tax Credit Rate, Percent							22.00%
Consolidated Appropriations Act of 2016; After 2021 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Investment Tax Credit Rate, Percent							10.00%
Tax Increase Prevention Act of 2014; 2014 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Bonus depreciation rate, Percent									50.00%
Tax Increase Prevention Act of 2014; 2015 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Bonus depreciation rate, Percent							50.00%