Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Summary of Financial Assets Recognized at Fair Value on Recurring Basis

As of December 31, 2018 and 2017, the Company’s financial assets recognized at fair value on a recurring basis consisted of the following:

	Fair Value as of December 31, 2018
	Total		Level 1		Level 2		Level 3
	(In thousands)
Cash equivalents	$	45,986	$	45,986	$	-	$	-
Marketable securities		255,203		165,948		89,255		-
Total	$	301,189	$	211,934	$	89,255	$	-

	Fair Value as of December 31, 2017
	Total		Level 1		Level 2		Level 3
	(In thousands)
Cash equivalents	$	330,896	$	330,896	$	-	$	-
Total	$	330,896	$	330,896	$	-	$	-

Summary of Available-for-sale Marketable Securities

The following table summarizes the Company’s available-for-sale marketable securities as of December 31, 2018 at net book value:

	December 31, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
	(In thousands)
Short-term marketable securities:
U.S. Treasury securities	$	165,959	$	2	$	(13	)	$	165,948
Financial institution debt securities		65,436		1		(17	)		65,420
Corporate debt securities		23,836		-		(1	)		23,835
Total	$	255,231	$	3	$	(31	)	$	255,203

Summary of Property and Equipment at Cost and Recognizes Depreciation and Amortization Using the Straight-Line Method Over Estimated Useful Lives

The Company records property and equipment at cost and recognizes depreciation and amortization using the straight-line method over the following estimated useful lives of the respective assets:

Asset Category	Useful Life
Laboratory equipment	5 years
Office furniture and equipment	5 years
Computer software	3 years
Computer equipment	3 years
Leasehold improvements	5 years or term of respective lease, if shorter

Summary of Changes in Contract Liabilities

The following table presents changes in the Company’s contract liabilities during the year ended December 31, 2018 (in thousands):

	Balance at Beginning of Period		Additions		Deductions			Balance at End of Period
Year Ended December 31, 2018
Contract liabilities:
Deferred revenue	$	59,868	$	19,000	$	(22,936	)	$	55,932

Summary of Revenues Recognized Resulting From Changes in Contract Liability Balance

During the year ended December 31, 2018, the Company recognized the following revenues as a result of changes in the contract liability balance (in thousands):

Revenue recognized in the period from:	Year Ended December 31, 2018
Amounts included in the contract liability at the beginning of the period	$	22,936

Schedule of Adjustments to Consolidated Balance Sheet Due to Adoption of New Guidance

As a result of applying the modified retrospective method to adopt the new guidance, the following adjustments were made to accounts on the consolidated balance sheet as of January 1, 2018:

Consolidated Balance Sheet	January 1, 2018 (in thousands)
	Pre-Adoption			ASC 606 Adjustment			Post-Adoption
Current portion of deferred revenue	$	21,188		$	(2,769	)	$	18,419
Deferred revenue, net of current portion		44,111			(2,662	)		41,449
Accumulated deficit		(121,113	)		5,431			(115,682	)

Summary of Impact of Adoption to Consolidated Statement of Operations and Balance Sheet

	Year Ended December 31, 2018
	Impact of changes in accounting policies
	As Reported			Balances without adoption of ASC 606			Effect of Change Higher/(Lower)
Collaboration revenue	$	30,434		$	31,838		$	(1,404	)
Operating loss		(90,870	)		(89,466	)		(1,404	)
Net loss	$	(85,343	)	$	(83,939	)	$	(1,404	)
Net loss per share, basic and diluted	$	(1.98	)	$	(1.95	)	$	(0.03	)
	December 31, 2018
	Impact of changes in accounting policies
	As Reported			Balances without adoption of ASC 606			Effect of Change Higher/(Lower)
Liabilities:
Deferred revenue – current	$	27,122		$	31,149		$	(4,027	)
Deferred revenue – noncurrent		28,810			28,810			-
Stockholders' equity:
Accumulated deficit	$	(201,025	)	$	(205,052	)	$	4,027