Marketable Securities - Narrative (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2017 USD ($) security	Dec. 31, 2016 USD ($) security
Investments, Debt and Equity Securities [Abstract]
Debt securities held in an unrealized loss position for more than 12 months | security	0	0
Debt securities held in an unrealized loss position for less than 12 months | security	3	13
Available-for-sale securities, accumulated gross unrealized losses (less than $1 thousand in 2017) | $	$ 0	$ 6
Fair value of debt securities in an unrealized loss position for less than 12 months | $	$ 9,400	$ 35,700
Marketable securities, contractual term to maturity	1 year