3. Fair Value Measurements

The Company’s cash equivalents are generally 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 generally based on prices obtained from independent pricing sources. Consistent with the fair value hierarchy described above, securities with validated quotes from pricing services are generally 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, 2016 and December 31, 2015:

	September 30, 2016
	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
	(in thousands)
Money market funds:
Money market funds	$	320,078	$	320,078	$	—	$	—
Total money market funds		320,078		320,078		—		—
Marketable securities:
U.S. government securities		24,050		—		24,050		—
Corporate bonds		64,672		—		64,672		—
Commercial paper		22,470		—		22,470		—
Total marketable securities		111,192		—		111,192		—
Total money market funds and marketable securities	$	431,270	$	320,078	$	111,192	$	—

	December 31, 2015
	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
	(in thousands)
Cash and cash equivalents:
Money market funds	$	186,753	$	186,753	$	—	$	—
Total cash and cash equivalents	$	186,753	$	186,753	$	—	$	—

During the three and nine months ended September 30, 2016 and 2015, there were no transfers among the Level 1, Level 2 and Level 3 categories.

Marketable Securities

The following table summarizes the Company’s marketable securities as of September 30, 2016:

	September 30, 2016
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in thousands)
Assets:
U.S. government securities (due within 1 year)	$	24,037	$	13	$	—		$	24,050
Corporate bonds (due within 1 year)		64,720		4		(52	)		64,672
Commercial paper (due within 1 year)		22,470		—		—			22,470
	$	111,227	$	17	$	(52	)	$	111,192

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, 2016 and 2015. The Company held no marketable securities as of December 31, 2015.