The Tax Act was enacted on December 22, 2017 and includes a number of changes to existing tax laws that impact us, most notably it reduces the US federal corporate tax rate from 35% to 21%, for tax years beginning after December 31, 2017. The Tax Act made modifications to allowable tax depreciation, the deductability of compensation for officers, the deductibility of meals and entertainment expenses, and the deductibility of interest expense.


	Year Ended December 31,
	2019		2018		2017
Current expense (benefit):
Federal	$	89,471	$	—	$	—
State	3,103		424		111
Foreign	(66)		(158)		261
	92,508		266		372
Deferred expense (benefit):
Federal	74,627		29,928		44,075
State	202		(185)		228
	$	167,337	$	30,009	$	44,675

A reconciliation of income tax expense (benefit) from continuing operations to the amount computed by applying the statutory federal income tax rate to the net income (loss) from continuing operations is summarized as follows (in thousands):


	Year Ended December 31,
	2019		2018		2017
Tax at federal statutory rate	$	167,294	$	36,400	$	20,031
State, net of federal benefit	2,466		1,635		622
Contingent liabilities	18		948		903
Share-based compensation	(819)		(8,131)		(4,019)
FDII	(402)		—		—
Research and development credits	(879)		(2,758)		(2,821)
Change in uncertain tax positions	441		858		1,308
Rate change for changes in federal or state law	(210)		178		32,429
Change in valuation allowance	(1,193)		(4,225)		(4,169)
Expired NOLs and credits	—		3,054		—
Change in derivatives	—		615		—
Other	621		1,435		391
	$	167,337	$	30,009	$	44,675

We remeasured certain deferred tax assets and liabilities based on the rates at which they are expected to reverse in the future, which is generally 21%. Significant components of our deferred tax assets and liabilities as of December 31, 2019 and 2018 are shown below. We assess the positive and negative evidence to determine if sufficient future taxable income will be generated to use the existing deferred tax assets. Our evaluation of evidence resulted in management concluding that the majority of our deferred tax assets will be realized. However, we maintain a valuation allowance to offset certain net deferred tax assets as management believes realization of such assets are uncertain as of December 31, 2019, 2018 and 2017. The valuation allowance increased $136.9 million in 2019, decreased $2.5 million in 2018 and decreased $8.4 million in 2017.

We offset all deferred tax assets and liabilities by jurisdiction, as well as any related valuation allowance, and present them on our consolidated balance sheet as a non-current deferred income tax asset or liability (as applicable). Deferred tax assets (liabilities) are comprised of the following:


	December 31,
	2019		2018
	(in thousands)
Deferred tax assets:
Net operating loss carryforwards	$	150,727	$	57,181
Research credit carryforwards	14,843		31,101
Fixed assets and intangibles	419		1,637
Accrued expenses	171		657
Deferred revenue	—		957
Other	17,960		11,430
	184,120		102,963
Valuation allowance for deferred tax assets	(141,338)		(4,476)
Net deferred tax assets	$	42,782	$	98,487

Deferred tax liabilities:
Retrophin fair value adjustment	(18)		(179)
Convertible debt	—		(2,905)
Identified intangibles	(41,664)		(44,643)
Identified indefinite lived intangibles	(1,040)		(1,759)
Investment in Viking	(2,937)		(2,480)
Deferred revenue	(3,488)		—
Other	(964)		—
Net deferred tax liabilities	$	(50,111)	$	(51,966)

Deferred income taxes, net	$	(7,329)	$	46,521

As of December 31, 2019, we had federal net operating loss carryforwards set to expire through 2037 of $31.5 million and $119.1 million of state net operating loss carryforwards that begin to expire in 2028. We also have $0.6 million of federal research and development credit carryforwards, which expire through 2038. We have $22.9 million of California research and development credit carryforwards that have no expiration date. In addition, we have approximately $713.8 million of non-U.S. net operating loss carryovers and approximately $14.6 million of non-U.S. capital loss carryovers that have no expiration date. We have a full valuation allowance against these non-U.S. tax attributes.

Pursuant to Section 382 and 383 of the Internal Revenue Code of 1986, as amended, utilization of our net operating losses and credits may be subject to annual limitations in the event of any significant future changes in its ownership structure. These annual limitations may result in the expiration of net operating losses and credits prior to utilization. The deferred tax assets as of December 31, 2019 are net of any previous limitations due to Section 382 and 383.

We account for income taxes by evaluating a probability threshold that a tax position must meet before a financial statement benefit is recognized. The minimum threshold is a tax position that is more likely than not to be sustained upon examination by the applicable taxing authority, including resolution of any related appeals or litigation processes, based on the technical merits of the position. Our remaining liabilities for uncertain tax positions are presented net of the deferred tax asset balances on the accompanying consolidated balance sheet.


	December 31,
	2019		2018		2017
Balance at beginning of year	$	30,289	$	29,363	$	38,770
Additions based on tax positions related to the current year	543		1,247		1067
Additions for tax positions of prior years	(54)		336		109
Reductions for tax positions of prior years	(2,042)		(657)		(10,583)
Balance at end of year	$	28,736	$	30,289	$	29,363

Included in the balance of unrecognized tax benefits at December 31, 2019 is $27.1 million of tax benefits that, if recognized would impact the effective rate. There are no positions for which it is reasonably possible that the uncertain tax benefit will significantly increase or decrease within twelve months.

We recognize interest and penalties related to uncertain tax positions in income tax expense. As of December 31, 2019 and December 31, 2018, we recognized an immaterial amount of interest and penalties. We file income tax returns in the United States, various state jurisdictions, United Kingdom, and Canada with varying statutes of limitations. The federal statute of limitation remains open for the 2016 tax year to the present. The state income tax returns generally remain open for the 2015 tax year through the present. Net operating loss and research credit carryforwards arising prior to these years are also open to examination if and when utilized.

We are subject to taxation in the U.S. and various states and foreign jurisdictions. With few exceptions, as of December 31, 2019, we are no longer subject to state, local or foreign examinations by tax authorities for tax years before 2015 and we are no longer subject to U.S. federal income or payroll tax examinations for tax years before 2017. No tax returns are currently under examination by any tax authorities. Net operating loss and research credit carryforwards arising prior to these years are also open to examination if and when utilized. We believe our reserve for unrecognized tax benefits and contingent tax issues is adequate with respect to all open years.