Income tax expense attributable to earnings before income taxes consists of (in thousands):

Income tax expense attributable to earnings before income taxes differed from the amounts computed using the statutory federal income tax rate of 21% as follows (in thousands):

The Tax Cuts and Jobs Act (the Act) was enacted in December 2017. Beginning in 2018, the Act reduced the U.S. federal corporate tax rate from 35% to 21%. At December 31, 2017, we had not completed our accounting for the tax effects of enactment of the Act. However, we made a reasonable estimate of the effects on our existing deferred tax assets and liabilities based on the rates at which they were expected to reverse in the future, which was generally 21%. The provisional amount recorded resulting from the remeasurement of our deferred tax balance was $309.2 million, which was included as a component of 2017 income tax from continuing operations. During 2018, we finalized our calculations for our 2017 federal income tax return, which was filed based on the law prior to the Act, resulting in no significant change to the initial measurement of these balances. Remaining aspects of the Act were not relevant to our operations.

Income taxes receivable was $60.9 million and $102.4 million at December 31, 2019 and 2018, respectively. These amounts have been included in other receivables in our Consolidated Balance Sheets. The tax effects of temporary differences that give rise to significant portions of the deferred tax assets and deferred tax liabilities at December 31, 2019 and 2018, are presented below (in thousands):

	December 31,
	2019			2018
Deferred tax assets:
Insurance accruals	$	27,180		$	34,889
Allowance for doubtful accounts		8,052			7,649
Compensation accrual		4,925			10,461
Deferred compensation accrual		24,521			20,396
Federal benefit of state uncertain tax positions		9,867			10,364
Lease liabilities		30,251			-
State NOL carry-forward		7,495			6,041
Other		6,357			4,626
Total gross deferred tax assets		118,648			94,426
Valuation allowance		(7,495	)		(6,041	)
Total deferred tax assets, net of valuation allowance		111,153			88,385
Deferred tax liabilities:
Plant and equipment, principally due to differences in depreciation		729,016			696,913
Prepaid permits and insurance, principally due to expensing for income tax purposes		39,285			33,594
Lease right-of-use assets		30,014			-
Other		11,916			1,339
Total gross deferred tax liabilities		810,231			731,846
Net deferred tax liability	$	699,078		$	643,461

Guidance on accounting for uncertainty in income taxes prescribes recognition and measurement criteria and requires that we assess whether the benefits of our tax positions taken are more likely than not of being sustained under tax audits. We have made adjustments to the balance of unrecognized tax benefits, a component of other long-term liabilities on our Consolidated Balance Sheets, as follows (in millions):

	December 31,
	2019			2018			2017
Beginning balance	$	52.2		$	45.3		$	35.4
Additions based on tax positions related to the current year		11.0			13.9			11.6
Additions/(reductions) based on tax positions taken in prior years		(6.5	)		(2.4	)		5.4
Reductions due to settlements		-			-			(2.4	)
Reductions due to lapse of applicable statute of limitations		(6.1	)		(4.6	)		(4.7	)
Ending balance	$	50.6		$	52.2		$	45.3

At December 31, 2019 and 2018, we had a total of $50.6 million and $52.2 million, respectively, in gross unrecognized tax benefits. Of these amounts, $41.8 million and $43.1 million represent the amount of unrecognized tax benefits that, if recognized, would impact our effective tax rate in 2019 and 2018, respectively. Interest and penalties related to income taxes are classified as interest expense in our Consolidated Statements of Earnings. The amount of accrued interest and penalties recognized during the years ended December 31, 2019, 2018, and 2017, was $3.2 million, $2.4 million, and $2.1 million, respectively. Future changes to unrecognized tax benefits will be recognized as income tax expense and interest expense, as appropriate. The total amount of accrued interest and penalties for such unrecognized tax benefits at December 31, 2019 and 2018, was $4.8 million and $4.6 million, respectively.

Tax years 2016 and forward remain subject to examination by federal tax jurisdictions, while tax years 2009 and forward remain open for state jurisdictions.