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 September 30, 2017 and December 31, 2016:

	September 30, 2017
	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
	(in thousands)
Cash equivalents:
Money market funds	$	134,916	$	134,916	$	—	$	—
Total cash equivalents		134,916		134,916		—		—
Marketable securities:
U.S. government securities		37,992		—		37,992		—
U.S. corporate bonds		16,317		—		16,317		—
U.S. commercial paper		24,710		—		24,710		—
International commercial paper		29,516		—		29,516		—
Total marketable securities		108,535		—		108,535		—
Total cash equivalents and marketable securities	$	243,451	$	134,916	$	108,535	$	—

	December 31, 2016
	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
	(in thousands)
Cash equivalents:
Money market funds	$	168,517	$	168,517	$	—	$	—
Total cash equivalents		168,517		168,517		—		—
Marketable securities:
U.S. government securities		100,031		—		100,031		—
U.S. corporate bonds		58,452		—		58,452		—
International corporate bonds		24,190		—		24,190		—
U.S. commercial paper		30,351		—		30,351		—
International commercial paper		15,938		—		15,938		—
Total marketable securities		228,962		—		228,962		—
Total cash equivalents and marketable securities	$	397,479	$	168,517	$	228,962	$	—

During the nine months ended September 30, 2017 and 2016, 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 September 30, 2017 and December 31, 2016:

	September 30, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in thousands)
Assets:
U.S. government securities	$	38,003	$	—	$	(11	)	$	37,992
U.S. corporate bonds		16,321		1		(5	)		16,317
U.S. commercial paper		24,710		—		—			24,710
International commercial paper		29,516		—		—			29,516
	$	108,550	$	1	$	(16	)	$	108,535

	December 31, 2016
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in thousands)
Assets:
U.S. government securities	$	100,055	$	2	$	(26	)	$	100,031
U.S. corporate bonds		58,508		—		(56	)		58,452
International corporate bonds		24,212		—		(22	)		24,190
U.S. commercial paper		30,351		—		—			30,351
International commercial paper		15,938		—		—			15,938
	$	229,064	$	2	$	(104	)	$	228,962

As of September 30, 2017, all marketable securities 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 and nine months ended September 30, 2017 and the year ended December 31, 2016.