Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax [Line Items]
Period of Average Annual Book Income Before Taxes			3 years
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax			$ 8,855	$ 16,620
Federal statutory tax rate			21.00%	35.00%	35.00%
Deferred Income Tax Expense (Benefit)			$ 29,118	$ 49,394	$ 50,510
Deferred Tax Liabilities, Regulatory Assets			74,848	96,036
Deferred Tax Liabilities, Regulatory Assets and Liabilities			298,570	296,685
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions			200	0	0
Unrecognized tax benefits that would impact effective tax rate			1,600
Unrecognized tax benefits, interest on income taxes expense (benefit)			600	(200)	100
Unrecognized tax benefits, income tax penalties and interest accrued			$ 1,200	$ 700
Internal Revenue Service (IRS)
Income Tax [Line Items]
Operating loss carryforward, Limitations on Use, Percent			80.00%	100.00%
Tax Credit Carryforward, Amount			$ 11,900
Live Of Tax Loss Carrforwards			20 years
State and Local Jurisdiction
Income Tax [Line Items]
Regulatory Asset, Amortization Period			15 years
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions			$ 500
Tax Credit Carryforward, Amount			800
Impact due to the Tax Cuts and Jobs Act of 2017 [Domain]
Income Tax [Line Items]
ADFIT				$ 298,900
Deferred Tax Liabilities, Regulatory Assets				23,600
Deferred Tax Liabilities, Regulatory Assets and Liabilities				275,300
Tax credit taken | ARIZONA
Income Tax [Line Items]
Unrecognized Tax Benefits, Period Increase (Decrease)			$ 500	100	(300)
Earliest Tax Year | TEXAS
Income Tax [Line Items]
Tax Year Closed			2007
Latest Tax Year | Internal Revenue Service (IRS)
Income Tax [Line Items]
Tax Year Closed			2013
Latest Tax Year | NEW MEXICO
Income Tax [Line Items]
Tax Year Closed			2013
Latest Tax Year | TEXAS
Income Tax [Line Items]
Tax Year Closed			2011
Latest Tax Year | ARIZONA
Income Tax [Line Items]
Tax Year Closed			2013
Change in Accounting Method Accounted for as Change in Estimate
Income Tax [Line Items]
Deferred Income Tax Expense (Benefit)			$ 2,300	1,900	5,100
Scenario, Forecast
Income Tax [Line Items]
Percent of refund allowed to claim of the remaining alternative minimum tax	50.00%	50.00%
Electric Transmission and Distribution Equipment | TEXAS
Income Tax [Line Items]
Unrecognized Tax Benefits, Period Increase (Decrease)				(1,200)	(400)
Depreciation | TEXAS
Income Tax [Line Items]
Unrecognized Tax Benefits, Period Increase (Decrease)				$ 500	$ 300