Fair Value Measurements (Tables)

9 Months Ended

Sep. 30, 2018

Fair Value Disclosures [Abstract]

Summary of Company's Money Market Funds and Marketable Securities

The following tables summarize the Company’s money market funds and marketable securities as of September 30, 2018 and December 31, 2017.

	September 30, 2018
	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
	(in thousands)
Cash equivalents:
Cash equivalents	$	253,123	$	253,123	$	—	$	—
Total cash equivalents		253,123		253,123		—		—
Marketable securities:
U.S. government securities		283,570		—		283,570		—
U.S. corporate bonds		152,747		—		152,747		—
International corporate bonds		89,551		—		89,551		—
U.S. commercial paper		103,531		—		103,531		—
International commercial paper		138,879		—		138,879		—
Total marketable securities		768,278		—		768,278		—
Total cash equivalents and marketable securities	$	1,021,401	$	253,123	$	768,278	$	—

	December 31, 2017
	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
	(in thousands)
Cash equivalents:
Cash equivalents	$	306,235	$	306,235	$	—	$	—
Total cash equivalents		306,235		306,235		—		—
Marketable securities:
U.S. government securities		49,606		—		49,606		—
U.S. corporate bonds		48,959		—		48,959		—
U.S. commercial paper		65,583		—		65,583		—
International commercial paper		48,465		—		48,465		—
Total marketable securities		212,613		—		212,613		—
Total cash equivalents and marketable securities	$	518,848	$	306,235	$	212,613	$	—

Summary of Gross Unrealized Gains and Losses of Marketable Securities

The following tables summarize the gross unrealized gains and losses of the Company’s marketable securities as of September 30, 2018 and December 31, 2017:

	September 30, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in thousands)
Assets:
U.S. government securities	$	283,701	$	—	$	(131	)	$	283,570
U.S. corporate bonds		152,848		5		(106	)		152,747
International corporate bonds		89,610		1		(60	)		89,551
U.S. commercial paper		103,516		34		(19	)		103,531
International commercial paper		138,860		30		(11	)		138,879
	$	768,535	$	70	$	(327	)	$	768,278

	December 31, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in thousands)
Assets:
U.S. government securities	$	49,612	$	—	$	(6	)	$	49,606
U.S. corporate bonds		48,982		2		(25	)		48,959
U.S. commercial paper		65,583		—		—			65,583
International commercial paper		48,465		—		—			48,465
	$	212,642	$	2	$	(31	)	$	212,613