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Income Taxes (Tables)

12 Months Ended

Dec. 31, 2020

Income Tax Disclosure [Abstract]

Summary of Effective Income Tax Rates

The components of income before income taxes for each of the three years ended December 31, 2020 and income tax expense (benefit) attributable thereto were as follows:

			Years Ended December 31,
(Millions of dollars)			2020						2019						2018
Income (loss) before income taxes			$	509.1					$	202.4					$	273.9
Income tax expense (benefit)
Federal - Current			$	96.0					15.6						18.4
Federal - Deferred			4.7						21.7						31.0
State - Current and deferred			22.3						10.3						10.9
Total			$	123.0					$	47.6					$	60.3

Schedule of Reconciliation of Income Taxes to Statutory Rate

The following table reconciles income taxes based on the U.S. statutory tax rate to the Company’s income tax expense (benefit).

			Years Ended December 31,
(Millions of dollars)			2020						2019						2018
Income tax expense based on the U.S. statutory tax rate			$	106.9					$	42.5					$	57.5
State income taxes, net of federal benefit			17.5						8.6						8.3
Federal credits			(1.9)						(2.3)						(2.0)
Other, net			0.5						(1.2)						(3.5)
Total			$	123.0					$	47.6					$	60.3

Summary of Deferred Tax Assets and Deferred Tax Liabilities

An analysis of the Company’s deferred tax assets and deferred tax liabilities at December 31, 2020 and 2019 showing the tax effects of significant temporary differences is as follows:

			December 31,
(Millions of dollars)			2020						2019
Deferred tax assets
Property costs and asset retirement obligations			$	4.5					$	3.7
Employee benefits			8.0						6.1
Operating leases liability			31.6						25.0
Other deferred tax assets			7.2						2.1
Total gross deferred tax assets			51.3						36.9
Deferred tax liabilities
Accumulated depreciation and amortization			(213.2)						(191.2)
State deferred taxes			(20.2)						(27.9)
Operating leases right of use assets			(31.0)						(24.8)
Other deferred tax liabilities			(5.3)						(9.7)
Total gross deferred tax liabilities			(269.7)						(253.6)
Net deferred tax liabilities			$	(218.4)					$	(216.7)

Reconciliation of Beginning and Ending Liability for Uncertain Tax Positions

A reconciliation of the beginning and ending amount of the consolidated liability for unrecognized income tax benefits during the year ended December 31, 2020 and 2019 is shown in the following table.  

			Year Ended December 31,
(Millions of dollars)			2020						2019
Balance at January 1			$	0.6					$	0.7
Additions for tax positions related to prior years			0.1						0.5
Settlements with taxing authorities			(0.3)						(0.6)
Balance at December 31			$	0.4					$	0.6

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Tabular disclosure of the components of income tax expense attributable to continuing operations for each year presented including, but not limited to: current tax expense (benefit), deferred tax expense (benefit), investment tax credits, government grants, the benefits of operating loss carryforwards, tax expense that results from allocating certain tax benefits either directly to contributed capital or to reduce goodwill or other noncurrent intangible assets of an acquired entity, adjustments of a deferred tax liability or asset for enacted changes in tax laws or rates or a change in the tax status of the entity, and adjustments of the beginning-of-the-year balances of a valuation allowance because of a change in circumstances that causes a change in judgment about the realizability of the related deferred tax asset in future years.

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Tabular disclosure of the components of net deferred tax asset or liability recognized in an entity's statement of financial position, including the following: the total of all deferred tax liabilities, the total of all deferred tax assets, the total valuation allowance recognized for deferred tax assets.

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Tabular disclosure of the reconciliation using percentage or dollar amounts of the reported amount of income tax expense attributable to continuing operations for the year to the amount of income tax expense that would result from applying domestic federal statutory tax rates to pretax income from continuing operations.

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