Fair Value Measurements (Tables)

6 Months Ended

Jun. 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 June 30, 2018 and December 31, 2017.

	June 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	$	325,830	$	325,830	$	—	$	—
Total cash equivalents		325,830		325,830		—		—
Marketable securities:
U.S. government securities		225,335		—		225,335		—
U.S. corporate bonds		110,363		—		110,363		—
International corporate bonds		26,503		—		26,503		—
U.S. commercial paper		211,144		—		211,144		—
International commercial paper		193,258		—		193,258		—
Total marketable securities		766,603		—		766,603		—
Total cash equivalents and marketable securities	$	1,092,433	$	325,830	$	766,603	$	—

	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 June 30, 2018 and December 31, 2017:

	June 30, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in thousands)
Assets:
U.S. government securities	$	225,363	$	5	$	(33	)	$	225,335
U.S. corporate bonds		110,454		1		(92	)		110,363
International corporate bonds		26,518		—		(15	)		26,503
U.S. commercial paper		211,150		—		(6	)		211,144
International commercial paper		193,263		—		(5	)		193,258
	$	766,748	$	6	$	(151	)	$	766,603

	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