Investments and Fair Value Measurements - Gross Unrealized Losses and Fair Value of Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Short-term investments:
Short-term investments, fair value	$ 222,205	$ 195,534
Short-term investments, gross unrealized loss	(1,033)	(464)
Cash equivalents:
Total cash equivalents and short-term investments in unrealized loss position, fair value	281,450	276,414
Total cash equivalents and short-term investments in unrealized loss position, gross unrealized loss	(1,033)	(467)
U.S. government securities
Short-term investments:
Short-term investments, fair value	22,440	17,431
Short-term investments, gross unrealized loss	(94)	(41)
U.S. government agency securities
Short-term investments:
Short-term investments, fair value	14,290	11,510
Short-term investments, gross unrealized loss	(56)	(30)
Municipal bonds
Short-term investments:
Short-term investments, fair value	3,544	6,968
Short-term investments, gross unrealized loss	(4)	(8)
Foreign government securities
Short-term investments:
Short-term investments, fair value	2,498	7,472
Short-term investments, gross unrealized loss	(6)	(27)
Corporate debt securities
Short-term investments:
Short-term investments, fair value	133,355	104,874
Short-term investments, gross unrealized loss	(685)	(273)
Commercial paper
Short-term investments:
Short-term investments, fair value	12,950	30,790
Short-term investments, gross unrealized loss	(4)	(9)
Asset-backed securities
Short-term investments:
Short-term investments, fair value	31,870	15,234
Short-term investments, gross unrealized loss	(184)	(76)
Commercial paper
Cash equivalents:
Cash equivalents, fair value	10,437	21,927
Cash equivalents, gross unrealized loss	0	(3)
Cash equivalents
Cash equivalents:
Cash equivalents, fair value	59,245	80,880
Cash equivalents, gross unrealized loss	0	(3)
Short-term investments
Short-term investments:
Short-term investments, fair value	212,784	189,213
Short-term investments, gross unrealized loss	(1,033)	(464)
Short-term investments | U.S. government securities
Short-term investments:
Short-term investments, fair value	22,440	17,431
Short-term investments, gross unrealized loss	(94)	(41)
Short-term investments | U.S. government agency securities
Short-term investments:
Short-term investments, fair value	14,290	11,510
Short-term investments, gross unrealized loss	(56)	(30)
Short-term investments | Municipal bonds
Short-term investments:
Short-term investments, fair value	3,544	5,767
Short-term investments, gross unrealized loss	(4)	(8)
Short-term investments | Foreign government securities
Short-term investments:
Short-term investments, fair value	2,498	7,472
Short-term investments, gross unrealized loss	(6)	(27)
Short-term investments | Corporate debt securities
Short-term investments:
Short-term investments, fair value	125,192	103,869
Short-term investments, gross unrealized loss	(685)	(273)
Short-term investments | Commercial paper
Short-term investments:
Short-term investments, fair value	12,950	27,930
Short-term investments, gross unrealized loss	(4)	(9)
Short-term investments | Asset-backed securities
Short-term investments:
Short-term investments, fair value	31,870	15,234
Short-term investments, gross unrealized loss	$ (184)	(76)
Cash equivalents | Commercial paper
Cash equivalents:
Cash equivalents, fair value		19,982
Cash equivalents, gross unrealized loss		(3)
Cash equivalents | Cash equivalents
Cash equivalents:
Cash equivalents, fair value		19,982
Cash equivalents, gross unrealized loss		(3)
Cash equivalents and short-term investments
Cash equivalents:
Total cash equivalents and short-term investments in unrealized loss position, fair value		209,195
Total cash equivalents and short-term investments in unrealized loss position, gross unrealized loss		$ (467)