Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Amortized Cost
Amortized cost	$ 83,461	$ 99,171
Gross Unrealized Gains
Gross Unrealized Gains	2	4
Gross Unrealized Losses
Gross Unrealized Losses	(39)	(36)
Fair Value
Fair Value	83,424	99,139
Other-than-temporarily impairment	0	0
Accrued interest	200	300
U.S. government and municipal obligations
Amortized Cost
Maturing in one year or less	24,150	26,164
Gross Unrealized Gains
Maturing in one year or less	2	3
Gross Unrealized Losses
Maturing in one year or less	(7)	(9)
Fair Value
Maturing in one year or less	24,145	26,158
Corporate debt securities
Amortized Cost
Maturing in one year or less	59,311	73,007
Gross Unrealized Gains
Maturing in one year or less		1
Gross Unrealized Losses
Maturing in one year or less	(32)	(27)
Fair Value
Maturing in one year or less	$ 59,279	$ 72,981