
	Year Ended December 31,
	2020		2019		2018
Current:
Federal	$	2	$	—	$	—
State	(10)		2		1
Foreign	—		—		1
Total current	(8)		2		2
Deferred:
Federal	(7)		(4)		(1)
State	(3)		(4)		(25)
Foreign	—		(1)		(5)
Total deferred	(10)		(9)		(31)
Total income tax benefit	$	(18)	$	(7)	$	(29)

The decrease in the effective tax rate for the year ended December 31, 2020, compared to the year ended December 31, 2019 is primarily due to the to the combined effects of (i) the impact of a smaller gain on the Newhurst and Protos transaction in 2020 compared to gain on the Rookery transaction in 2019, (ii) a discrete tax benefit adjustment in 2020 related to tax carryforwards and (iii) the discrete tax benefit related to the release of State FIN 48 reserve for uncertain tax position.

A reconciliation of our income tax (benefit) expense at the federal statutory income tax rate of 21% to our income tax benefit at the effective tax rate is as follows (in millions):


	Year Ended December 31,
	2020		2019		2018
Income tax (benefit) expense at the federal statutory rate	$	(10)	$	(1)	$	25
State and other tax benefit	(2)		(1)		(1)
Tax rate differential on foreign earnings	(1)		(2)		(3)

Gain on sale of business	(4)		(9)		(44)
Permanent differences	2		4		5


Impact of state apportionment & tax rate	1		(2)		(13)

Change in valuation allowance	2		1		3
Liability for uncertain tax positions	(9)		(1)		(4)


Tax reform transition tax	—		—		1

Other	3		4		2
Total income tax benefit	$	(18)	$	(7)	$	(29)

We had consolidated federal net operating loss carryforwards (“NOLs”) estimated to be approximately $239 million for federal income tax purposes as of December 31, 2020. The majority of these NOLs will expire in 2033 and beyond, if not used.

In addition to the consolidated federal NOLs, as of December 31, 2020, we had state NOL carryforwards of approximately $508 million, which expire between 2030 and 2039, net foreign NOL carryforwards of approximately $181 million with some expiring between 2021 and 2039. The federal tax credit carryforwards include production tax credits of $60 million expiring between 2024 and 2040, and research and experimentation tax credits of $1 million expiring between 2027 and 2033. Additionally, we had state income tax credits of $1 million.

The tax effects of temporary differences that give rise to the deferred tax assets and liabilities are presented as follows (in millions):


	As of December 31,
	2020		2019
Deferred tax assets:


NOLs	$	106	$	90
Accrued and prepaid expenses	67		63
Tax credits	49		49
Interest expense	14		26



Other	12		8

Total gross deferred tax asset	248		236
Less: valuation allowance	(72)		(65)
Total deferred tax asset	176		171
Deferred tax liabilities:

Property, plant and equipment	496		517
Intangible assets	29		17




Other, net	13		9
Total gross deferred tax liability	538		543
Net deferred tax liability	$	362	$	372

U.S. income taxes were not provided on cumulative undistributed foreign earnings as of December 31, 2020 and 2019. Foreign undistributed earnings were considered permanently invested, therefore no provision for U.S. income taxes was accrued as of December 31, 2020 and 2019.

Deferred tax assets relating to employee stock-based compensation deductions were reduced to reflect exercises of non-qualified stock option grants and vesting of restricted stock. Some exercises of non-qualified stock option grants and vesting of restricted stock resulted in tax deductions in excess of previously recorded benefits resulting in a "shortfall".


Balance at December 31, 2017	$	48
Additions based on tax positions related to the current year	2
Additions for tax positions of prior years	1
Reductions for lapse in applicable statute of limitations	(2)
Reductions for tax positions of prior years	(8)


Balance at December 31, 2018	41
Additions based on tax positions related to the current year	2

Reductions for lapse in applicable statute of limitations	(1)
Reductions for tax positions of prior years	(2)


Balance at December 31, 2019	40
Additions based on tax positions related to the current year	2

Reductions for lapse in applicable statute of limitations	(7)


Balance at December 31, 2020	$	35

The uncertain tax positions, exclusive of interest and penalties, were $35 million and $40 million as of December 31, 2020 and 2019, respectively, which also represent potential tax benefits that if recognized, would impact the effective tax rate.

We record interest accrued on liabilities for uncertain tax positions and penalties as part of the tax provision. As of December 31, 2020 and 2019, we had accrued interest and penalties associated with liabilities for uncertain tax positions of $1 million and $6 million, respectively.

Audits for federal income tax returns are closed for the years through 2010. However, the Internal Revenue Service ("IRS") can audit the NOLs generated during those years in the years that the NOLs are utilized.

State income tax returns are generally subject to examination for a period of three to six years after the filing of the respective tax return. The state impact of any federal changes remains subject to examination by various states for a period of up to one year after formal notification to the states. We have various state income tax returns in the process of examination, administrative appeals or litigation.