Fair Value Measurements

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Fair Value Measurements

3. Fair Value Measurements

The Company’s cash equivalents are classified within Level 1 of the fair value hierarchy.  The Company’s investments in marketable securities are classified within Level 2 of the fair value hierarchy.

The fair values of the Company’s marketable securities are based on prices obtained from independent pricing sources.  Consistent with the fair value hierarchy described above, securities with validated quotes from pricing services are reflected within Level 2, as they are primarily based on observable pricing for similar assets or other market observable inputs.  Typical inputs used by these pricing services include, but are not limited to, reported trades, benchmark yields, issuer spreads, bids, offers or estimates of cash flow, prepayment spreads and default rates.

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

	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	$	—

	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		—
	$	518,848	$	306,235	$	212,613	$	—

During the years ended December 31, 2018 and 2017, there were no transfers among the Level 1, Level 2 and Level 3 categories.

Marketable Securities

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

	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

	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

As of December 31, 2018, all marketable securities, except for two U.S. corporate bonds, held by the Company had remaining contractual maturities of one year or less.

As of December 31, 2018 and 2017, the Company held 145 and 11 marketable securities, respectively, that were in a loss position for less than one year due to fluctuations in interest rates.

There have been no impairments of the Company’s assets measured and carried at fair value during the years ended December 31, 2018 and 2017.