Fair Value Measurements (Tables)

9 Months Ended

Sep. 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 September 30, 2019 and December 31, 2018.

	September 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	$	259,181	$	259,181	$	—	$	—
Total cash equivalents		259,181		259,181		—		—
Marketable securities:
U.S. government securities		207,395		—		207,395		—
U.S. corporate bonds		395,804		—		395,804		—
International corporate bonds		157,780		—		157,780		—
U.S. commercial paper		53,541		—		53,541		—
International commercial paper		48,549		—		48,549		—
Total marketable securities		863,069		—		863,069		—
	$	1,122,250	$	259,181	$	863,069	$	—

	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 September 30, 2019 and December 31, 2018:

	September 30, 2019
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in thousands)
Assets:
U.S. government securities	$	207,183	$	217	$	(5	)	$	207,395
U.S. corporate bonds		394,825		1,045		(66	)		395,804
International corporate bonds		157,241		557		(18	)		157,780
U.S. commercial paper		53,505		36		—			53,541
International commercial paper		48,538		11		—			48,549
	$	861,292	$	1,866	$	(89	)	$	863,069

	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