Short-term Investments Short-term Investments - Summary of Investments in a Continuous Loss Position (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair value	$ 35,768
Less Than 12 Months, Gross unrealized loss	93
12 Months or Greater, Fair value	3,994
12 Months or Greater, Gross unrealized losses	6
Total, Fair value	39,762
Total, Gross unrealized losses	(99)
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair value	26,857
Less Than 12 Months, Gross unrealized loss	53
12 Months or Greater, Fair value	0
12 Months or Greater, Gross unrealized losses	0
Total, Fair value	26,857
Total, Gross unrealized losses	(53)
U.S. government-related debt securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair value	8,911
Less Than 12 Months, Gross unrealized loss	40
12 Months or Greater, Fair value	3,994
12 Months or Greater, Gross unrealized losses	6
Total, Fair value	12,905
Total, Gross unrealized losses	$ (46)