Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred Tax Assets
Benefit Liabilities	$ 41,292	$ 38,130
Retirement Benefits Liabilities	40,650	36,206
Tax Credit Carryforward	35,132	48,910
Regulatory Tax Liability	33,124	35,700
Cost of Removal	25,920	25,604
Differences Related to Property	7,486	6,979
Net Operating Loss Carryforward	1,379	1,475
Other	3,423	6,077
Valuation Allowance	(800)	(800)
Total Deferred Tax Assets	187,606	198,281
Deferred Tax Liabilities
Differences Related to Property	(271,064)	(268,495)
Retirement Benefits Regulatory Asset	(40,650)	(36,206)
Excess Tax Over Book Pension	(18,696)	(17,556)
Other	(10,572)	(7,965)
Total Deferred Tax Liabilities	(340,982)	(330,222)
Deferred Income Taxes	$ (153,376)	$ (131,941)

Amount, before allocation of valuation allowance, of deferred tax asset attributable to cost of removal.

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Amount, before allocation of valuation allowance, of deferred tax asset attributable to deductible temporary differences from pension and postretirement benefits.

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Amount, before allocation of valuation allowance, of deferred tax liability attributable to excess of tax over book related to pension.

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Represents an amount of deferred tax liability attributable to taxable temporary differences from retirement benefits regulatory assets.

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Amount of deferred tax liability attributable to taxable temporary differences.

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Amount after allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible operating loss carryforwards.

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Amount, before allocation of valuation allowance, of deferred tax asset attributable to deductible temporary differences, classified as other.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from property, plant, and equipment.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences derived regulatory assets and liabilities.

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Amount, before allocation of valuation allowance, of deferred tax asset attributable to deductible tax credit carryforwards, classified as other.

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Amount, before allocation of valuation allowance, of deferred tax asset attributable to deductible temporary differences from employee benefits, classified as other.

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Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

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Amount, after deferred tax asset, of deferred tax liability attributable to taxable differences without jurisdictional netting.

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Amount of deferred tax liability attributable to taxable temporary differences classified as other.

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Amount of deferred tax liability attributable to taxable temporary differences from property, plant, and equipment.

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