Fair Value Measurements (Tables)

6 Months Ended

Jun. 30, 2019

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, 2019 and December 31, 2018.

	June 30, 2019
	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	$	166,185	$	166,185	$	—	$	—
Total cash equivalents		166,185		166,185		—		—
Marketable securities:
U.S. government securities		290,571		—		290,571		—
U.S. corporate bonds		478,806		—		478,806		—
International corporate bonds		147,470		—		147,470		—
U.S. commercial paper		107,099		—		107,099		—
International commercial paper		46,418		—		46,418		—
Total marketable securities		1,070,364		—		1,070,364		—
	$	1,236,549	$	166,185	$	1,070,364	$	—

	December 31, 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	$	190,943	$	190,943	$	—	$	—
Total cash equivalents		190,943		190,943		—		—
Marketable securities:
U.S. government securities		220,482		—		220,482		—
U.S. corporate bonds		258,566		—		258,566		—
International corporate bonds		78,468		—		78,468		—
U.S. commercial paper		77,611		—		77,611		—
International commercial paper		96,706		—		96,706		—
Total marketable securities		731,833		—		731,833		—
	$	922,776	$	190,943	$	731,833	$	—

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, 2019 and December 31, 2018:

	June 30, 2019
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in thousands)
Assets:
U.S. government securities	$	290,271	$	302	$	(2	)	$	290,571
U.S. corporate bonds		477,932		942		(68	)		478,806
International corporate bonds		147,062		434		(26	)		147,470
U.S. commercial paper		107,105		1		(7	)		107,099
International commercial paper		46,419		—		(1	)		46,418
	$	1,068,789	$	1,679	$	(104	)	$	1,070,364

	December 31, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in thousands)
Assets:
U.S. government securities	$	220,531	$	8	$	(57	)	$	220,482
U.S. corporate bonds		258,876		6		(316	)		258,566
International corporate bonds		78,600		—		(132	)		78,468
U.S. commercial paper		77,630		8		(27	)		77,611
International commercial paper		96,711		8		(13	)		96,706
	$	732,348	$	30	$	(545	)	$	731,833