Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax [Line Items]
Adjustments to provisional estimates of the impact of the TCJA				12 months
Period of Average Annual Book Income Before Taxes				3 years
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax				$ 16,620	$ 16,620
Federal statutory tax rate				35.00%	35.00%	35.00%
Deferred Income Tax Expense (Benefit)				$ 49,394	$ 50,510	$ 30,846
Deferred Tax Liabilities, Regulatory Assets				96,036	118,861
Deferred Tax Liabilities, Regulatory Assets and Liabilities				296,685	18,435
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions				0	0	0
Unrecognized tax benefits that would impact effective tax rate				1,100
Unrecognized tax benefits, interest on income taxes expense (benefit)				(200)	100	200
Unrecognized tax benefits, income tax penalties and interest accrued				700	800
Accumulated Other Comprehensive Income (Loss), Net of Tax				$ 11,058	(7,116)
Internal Revenue Service (IRS)
Income Tax [Line Items]
Operating loss carryforward, Limitations on Use, Percent				100.00%
Tax Credit Carryforward, Amount				$ 23,000
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				$ 1,100
Tax Credit Carryforward, Amount				1,400
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)				$ 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				2012
Latest Tax Year | NEW MEXICO
Income Tax [Line Items]
Tax Year Closed				2012
Latest Tax Year | TEXAS
Income Tax [Line Items]
Tax Year Closed				2011
Latest Tax Year | ARIZONA
Income Tax [Line Items]
Tax Year Closed				2012
Change in Accounting Method Accounted for as Change in Estimate
Income Tax [Line Items]
Deferred Income Tax Expense (Benefit)				$ 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%	50.00%
Federal statutory tax rate			21.00%
Scenario, Forecast | Internal Revenue Service (IRS)
Income Tax [Line Items]
Operating loss carryforward, Limitations on Use, Percent			80.00%
Potential Impact of Adopting Accounting Standards Update 2018-02
Income Tax [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax				(7,200)
Electric Transmission and Distribution Equipment | TEXAS
Income Tax [Line Items]
Unrecognized Tax Benefits, Period Increase (Decrease)				(1,200)	(400)	$ (800)
Depreciation | TEXAS
Income Tax [Line Items]
Unrecognized Tax Benefits, Period Increase (Decrease)				$ 500	$ 300