Fair Value Measurements

3 Months Ended

Mar. 31, 2019

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 March 31, 2019 and December 31, 2018.

	March 31, 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	$	342,295	$	342,295	$	—	$	—
Total cash equivalents		342,295		342,295		—		—
Marketable securities:
U.S. government securities		248,155		—		248,155		—
U.S. corporate bonds		386,263		—		386,263		—
International corporate bonds		155,517		—		155,517		—
U.S. commercial paper		108,439		—		108,439		—
International commercial paper		110,582		—		110,582		—
Total marketable securities		1,008,956		—		1,008,956		—
	$	1,351,251	$	342,295	$	1,008,956	$	—

	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	$	—

During the three months ended March 31, 2019 and 2018, there were no transfers among the Level 1, Level 2 and Level 3 categories.

The following tables summarize the gross unrealized gains and losses of the Company’s marketable securities as of March 31, 2019 and December 31, 2018:

	March 31, 2019
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in thousands)
Assets:
U.S. government securities	$	248,082	$	86	$	(13	)	$	248,155
U.S. corporate bonds		386,333		119		(189	)		386,263
International corporate bonds		155,571		42		(96	)		155,517
U.S. commercial paper		108,487		5		(53	)		108,439
International commercial paper		110,589		—		(7	)		110,582
	$	1,009,062	$	252	$	(358	)	$	1,008,956

	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

As of March 31, 2019, all marketable securities, except for 58 bonds with a fair value of $166.2 million that have maturities of between one and two years, held by the Company had remaining contractual maturities of one year or less. 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.

There have been no impairments of the Company’s assets measured and carried at fair value during the three months ended March 31, 2019 and the year ended December 31, 2018.