Fair Value Measurements (Tables)

9 Months Ended

Sep. 30, 2017

Fair Value Disclosures [Abstract]

Summary of Company's Money Market Funds and Marketable Securities

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	$	—

Summary of Gross Unrealized Gains and Losses of 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