Income Taxes (Tables)

12 Months Ended

Dec. 31, 2017

A reconciliation of the Company’s effective tax rate to the statutory federal income tax rate is as follows:

	Years Ended December 31,
	2017			2016			2015
Federal income tax at statutory federal rate		35.0	%		35.0	%		35.0	%
State taxes		1.7			0.6			1.2
Permanent differences		(3.0	)		(6.3	)		(2.4	)
Stock-based compensation		(7.2	)		(1.5	)		(1.0	)
Tax Cuts and Jobs Act impact		(19.9	)		—			—
Tax credits		11.6			12.6			9.0
Change in deferred state tax rate		7.7			(0.3	)		(1.9	)
Other		0.3			3.7			(0.5	)
Change in valuation allowance		9.9			(36.0	)		(32.5	)
Total		36.1	%		7.8	%		6.9	%

Temporary differences that give rise to significant net deferred tax assets as of December 31, 2017 and 2016 are as follows:

	December 31,
(in thousands)	2017			2016
Deferred tax assets
Net operating losses	$	43,133		$	194,027
Capitalized research and development expenses		847			15,854
Credit carryforwards		164,709			144,823
Depreciation		2,605			2,557
Deferred compensation		5,855			12,463
Accrued expenses		905			3,601
Deferred revenue		—			21,935
Other temporary differences		2,145			25,276
Installment sale tax basis		15,868			—
Total gross deferred tax assets		236,067			420,536
Valuation allowance		(236,067	)		(414,558	)
Net deferred tax assets		—			5,978
Deferred tax liabilities
Intangible assets		—			(1,428	)
Debt discount		—			(4,550	)
Net deferred taxes	$	—		$	—

The change in the valuation allowance against the deferred tax assets in the years ended December 31, 2017, 2016 and 2015 was as follows:

(in thousands)	Balance at Beginning of Period		Additions		Deductions			Balance at End of Period
December 31, 2015	$	257,489	$	69,088	$	—		$	326,577
December 31, 2016		326,577		87,981		—			414,558
December 31, 2017		414,558		66,801		(245,292	)		236,067