Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Amortized Cost
Amortized cost	$ 72,809	$ 99,171
Gross Unrealized Gains
Gross Unrealized Gains		4
Gross Unrealized Losses
Gross Unrealized Losses	(14)	(36)
Fair Value
Fair Value	72,795	99,139
Number of investment-grade securities in unrealized loss position over 12 months	0	0
Accrued interest	100	300
U.S. government and municipal obligations
Amortized Cost
Maturing in one year or less	22,318	26,164
Gross Unrealized Gains
Maturing in one year or less		3
Gross Unrealized Losses
Maturing in one year or less	(7)	(9)
Fair Value
Maturing in one year or less	22,311	26,158
Corporate debt securities
Amortized Cost
Maturing in one year or less	50,491	73,007
Gross Unrealized Gains
Maturing in one year or less		1
Gross Unrealized Losses
Maturing in one year or less	(7)	(27)
Fair Value
Maturing in one year or less	$ 50,484	$ 72,981