Short-term Investments Short-term Investments - Summary of Investments in a Continuous Loss Position (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair value	$ 37,319
Less Than 12 Months, Gross unrealized loss	35
12 Months or Greater, Fair value	2,516
12 Months or Greater, Gross unrealized losses	6
Total, Fair value	39,835
Total, Gross unrealized losses	(41)
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair value	14,957
Less Than 12 Months, Gross unrealized loss	15
12 Months or Greater, Fair value	2,516
12 Months or Greater, Gross unrealized losses	6
Total, Fair value	17,473
Total, Gross unrealized losses	(21)
U.S. government-related debt securities
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair value	14,961
Less Than 12 Months, Gross unrealized loss	11
12 Months or Greater, Fair value	0
12 Months or Greater, Gross unrealized losses	0
Total, Fair value	14,961
Total, Gross unrealized losses	(11)
Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair value	7,401
Less Than 12 Months, Gross unrealized loss	9
12 Months or Greater, Fair value	0
12 Months or Greater, Gross unrealized losses	0
Total, Fair value	7,401
Total, Gross unrealized losses	$ (9)