Investments - Narrative (Details) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018 USD ($) security	Dec. 31, 2017
Debt Securities
Debt Securities, Available-for-sale [Line Items]
Number of securities in a continuous unrealized loss position for less than 12 months | security	2
Value of securities in a continuous unrealized loss position for less than 12 months | $	$ 2.9
Short-term Investments
Debt Securities, Available-for-sale [Line Items]
Weighted-average maturity of short-term investments	24 days	2 months
Short-term Investments | Minimum
Debt Securities, Available-for-sale [Line Items]
Maturity range for short-term investments	1 month	1 month
Short-term Investments | Maximum
Debt Securities, Available-for-sale [Line Items]
Maturity range for short-term investments	12 months	12 months