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 and/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 and/or estimates cash flow, prepayment spreads and default rates.

The following tables summarize the Company’s cash, cash equivalents and marketable securities as of June 30, 2016 and December 31, 2015:


	June 30, 2016
	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	$	189,003	$	189,003	$	—	$	—

Total cash and cash equivalents		189,003		189,003		—		—

Marketable securities:
U.S. government securities		32,082		—		32,082		—
Corporate bonds		28,788		—		28,788		—
Commercial paper		22,422		—		22,422		—

Total marketable securities		83,292		—		83,292		—

Total cash, cash equivalents and marketable securities	$	272,295	$	189,003	$	83,292	$	—


	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 six months ended June 30, 2016 and 2015, there were no transfers among the Level 1, Level 2 and Level 3 categories.

The carrying amounts reflected in the consolidated balance sheets for accounts payable and accrued expenses approximate fair value due to their short-term maturities.

Marketable Securities

The following table summarizes the Company’s marketable securities as of June 30, 2016.


	June 30, 2016
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in thousands)
Assets:
U.S. government securities (due within 1 year)	$	32,053	$	29	$	—		$	32,082
Corporate bonds (due within 1 year)		28,778		18		(8	)		28,788
Commercial paper (due within 1 year)		22,422		—		—			22,422

	$	83,253	$	47	$	(8	)	$	83,292

There have been no impairments of the Company’s assets measured and carried at fair value during the three and six months ended June 30, 2016 and 2015. The Company held no marketable securities as of December 31, 2015.