Cash, Cash Equivalents and Marketable Securities (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Cash and Cash Equivalents [Line Items]
Cash	$ 23,568,000		$ 23,568,000		$ 5,222,000
Total cash equivalents	9,531,000		9,531,000
Total cash and cash equivalents	33,099,000		33,099,000		68,454,000
Marketable securities, amortized cost	221,443,000		221,443,000		445,730,000
Marketable securities, gross unrealized gains	425,000		425,000		89,000
Marketable securities, gross unrealized losses	(3,000)		(3,000)		(86,000)
Marketable securities, estimated fair value	221,865,000		221,865,000		445,733,000
Total cash, cash equivalents and marketable securities, amortized cost	254,542,000		254,542,000		514,184,000
Total cash, cash equivalents and marketable securities	254,964,000		254,964,000		514,187,000
Amortized Cost
Due within one year, amortized cost	168,397,000		168,397,000
Due between one and two years, amortized cost	53,046,000		53,046,000
Marketable securities, amortized cost	221,443,000		221,443,000
Fair Value
Due within one year, estimated fair value	168,490,000		168,490,000
Due between one and two years, estimated fair value	53,375,000		53,375,000
Marketable securities, estimated fair value	221,865,000		221,865,000
Realized gain on sale of marketable securities	300,000	$ 0	322,000	$ 0
Commercial paper
Cash and Cash Equivalents [Line Items]
Total cash equivalents	996,000		996,000
Marketable securities, amortized cost	86,936,000		86,936,000		152,940,000
Marketable securities, gross unrealized gains	0		0		0
Marketable securities, gross unrealized losses	0		0		0
Marketable securities, estimated fair value	86,936,000		86,936,000		152,940,000
Money market funds
Cash and Cash Equivalents [Line Items]
Total cash equivalents	8,535,000		8,535,000		63,232,000
Corporate debt securities
Cash and Cash Equivalents [Line Items]
Marketable securities, amortized cost	81,835,000		81,835,000		60,622,000
Marketable securities, gross unrealized gains	425,000		425,000		58,000
Marketable securities, gross unrealized losses	(3,000)		(3,000)		(75,000)
Marketable securities, estimated fair value	82,257,000		82,257,000		60,605,000
Certificate of deposits
Cash and Cash Equivalents [Line Items]
Marketable securities, amortized cost	52,672,000		52,672,000		128,647,000
Marketable securities, gross unrealized gains	0		0		0
Marketable securities, gross unrealized losses	0		0		0
Marketable securities, estimated fair value	$ 52,672,000		$ 52,672,000		128,647,000
U.S. Treasury securities
Cash and Cash Equivalents [Line Items]
Marketable securities, amortized cost					103,521,000
Marketable securities, gross unrealized gains					31,000
Marketable securities, gross unrealized losses					(11,000)
Marketable securities, estimated fair value					$ 103,541,000