INCOME TAXES

The Provision for income taxes was based on the following income (loss) before income taxes (in thousands):



	Year Ended December 31,
	2017		2016			2015
Domestic	$	158,577	$	(70,222	)	$	(150,846	)
Foreign	—		—			(10,843		)
Income (loss) before income taxes	$	158,577	$	(70,222	)	$	(161,689	)

The Provision for income taxes was as follows (in thousands):



	Year Ended December 31,
	2017		2016		2015
Current:
Federal	$	—	$	—	$	—
State	4,350		—		55
Total current tax expense	4,350		—		55
Deferred:
Federal	—		—		—
State	—		—		—
Total deferred tax expense	—		—		—
Provision for income taxes	$	4,350	$	—	$	55

The Provision for income taxes for the year ended December 31, 2017 primarily relates to state taxes in jurisdictions outside of California, for which we do not have net operating loss carry-forwards due to a limited operating history. Our historical losses are sufficient to fully offset any federal taxable income. The Provision for income taxes for the year ended December 31, 2016 related to state minimum and franchise taxes and were nominal. The Provision for income taxes for the year ended December 31, 2015 relates to state minimum and franchise tax expenses as well as true ups related to prior year tax returns.

The reconciliation of income taxes at the statutory federal income tax rate to our Provision for income taxes included on the accompanying Consolidated Statements of Operations was as follows (in thousands):



	Year Ended December 31,
	2017			2016			2015
U.S. federal income tax provision (benefit) at statutory rate	$	53,916		$	(23,876	)	$	(54,974	)
Change in valuation allowance	(34,266		)	6,377			51,421
State tax expense	8,282			6,520			55
Debt extinguishment	—			4,726			—
Non-deductible interest	1,367			2,680			3,308
Stock-based compensation	(20,548		)	3,155			195
Other	(4,401		)	418			50
Provision for income taxes	$	4,350		$	—		$	55

Deferred tax assets and liabilities reflect the net tax effects of net operating loss and tax credit carry-forwards and temporary differences between the carrying amounts of assets and liabilities for financial reporting and the amounts used for income tax purposes.

Our deferred tax assets and liabilities were as follows (in thousands):



	December 31,
	2017			2016
Deferred tax assets:
Net operating loss carry-forwards	$	244,205		$	471,327
Book over tax depreciation and amortization	39,472			70,617
Tax credit and charitable contribution carry-forwards	66,770			64,367
Deferred revenue	53,543			—
Amortization of deferred stock compensation – non-qualified	8,966			14,780
Accruals and reserves not currently deductible	4,914			8,117
Other assets	1,088			106
Total deferred tax assets	418,958			629,314
Valuation allowance	(418,958		)	(629,062		)
Net deferred tax assets	—			252
Deferred tax liabilities:
Unrealized gains on derivatives and other liabilities	—			(252		)
Total deferred tax liabilities	—			(252		)
Net deferred taxes	$	—		$	—

On December 22, 2017, the Tax Cuts and Jobs Act of 2017 was signed into law. The Tax Cuts and Jobs Act contains significant changes to corporate taxation, including among other items, a reduction of the corporate tax rate from a top marginal rate of 35% to a flat rate of 21%. As a result of the signing of the Tax Cuts and Jobs Act, we recorded a $184.8 million reduction of our deferred tax assets along with a corresponding reduction of our valuation allowance. The Tax Cuts and Jobs Act could be amended or subject to technical correction, which could change the financial impacts that were recorded at December 31, 2017, or are expected to be recorded in future periods. Additionally, further guidance may be forthcoming from the FASB and the Securities and Exchange Commission, as well as regulations, interpretations and rulings from federal and state tax agencies, which could result in additional impacts.

ASC Topic 740 (“ASC 740”) requires that the tax benefit of net operating losses, temporary differences and credit carry forwards be recorded as an asset to the extent that management assesses that realization is "more likely than not." Realization of the future tax benefits is dependent on our ability to generate sufficient taxable income within the carry forward period. Because of our recent history of operating losses, management believes that recognition of the deferred tax assets arising from the above-mentioned future tax benefits is currently not likely (as defined in ASC 740) to be realized and, accordingly, has provided a valuation allowance. The valuation allowance decreased by $210.1 million during 2017 and increased by $92.7 million and $7.9 million during 2016 and 2015, respectively.

At December 31, 2017, we had federal net operating loss carry-forwards of approximately $1,105 million which expire in the years 2024 through 2036, and federal business tax credits of approximately $83 million which expire in the years 2020 through 2037. We also had state net operating loss carry-forwards of approximately $424 million, which expire in the years 2028 through 2036, and California research and development tax credits of approximately $28 million, which have no expiration.

Under the Internal Revenue Code and similar state provisions, certain substantial changes in our ownership could result in an annual limitation on the amount of net operating loss and credit carry-forwards that can be utilized in future years to offset future taxable income. The annual limitation may result in the expiration of net operating losses and credit carry-forwards before utilization. We completed a Section 382 study through December 31, 2017, and concluded that an ownership change, as defined under Section 382, had not occurred.

ASC 740 clarifies the accounting for uncertainty in income taxes by prescribing the recognition threshold a tax position is required to meet before being recognized in the financial statements. It also provides guidance on derecognition, classification, interest and penalties, accounting in interim periods, disclosure and transition. The following table summarizes the activity related to our unrecognized tax benefits (in thousands):



	Year Ended December 31,
	2017			2016			2015
Beginning balance	$	61,809		$	88,638		$	58,215
Change relating to prior year provision	247			(29,110		)	21,696
Change relating to current year provision	17,378			2,304			8,727
Reductions based on the lapse of the applicable statutes of limitations	(92		)	(23		)	—
Ending balance	$	79,342		$	61,809		$	88,638

We do not anticipate that the amount of unrecognized tax benefits existing as of December 31, 2017 will significantly decrease over the next 12 months.

We file U.S. and state income tax returns in jurisdictions with varying statues of limitations during which such tax returns may be audited and adjusted by the relevant tax authorities. The 1999 through 2016 tax years generally remain subject to examination by federal and most state tax authorities to the extent net operating losses and credits generated during these periods are being utilized in the open tax periods.