Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Amortized Cost
Amortized cost	$ 85,803	$ 147,370
Gross Unrealized Gains
Gross Unrealized Gains	8	13
Gross Unrealized Losses
Gross Unrealized Losses	(16)	(68)
Fair Value
Fair Value	85,795	147,315
Other-than-temporarily impairment	0
Accrued interest	300	600
U.S government and municipal obligations
Amortized Cost
Maturing in one year or less	13,725	52,754
Maturing after one year through three years		296
Amortized cost	13,725	53,050
Gross Unrealized Gains
Maturing in one year or less	5	5
Maturing after one year through three years		8
Gross Unrealized Gains	5	13
Gross Unrealized Losses
Maturing in one year or less	(3)	(12)
Gross Unrealized Losses	(3)	(12)
Fair Value
Maturing in one year or less	13,727	52,747
Maturing after one year through three years		304
Fair Value	13,727	53,051
Corporate debt securities
Amortized Cost
Maturing in one year or less	72,078	94,320
Amortized cost	72,078	94,320
Gross Unrealized Gains
Maturing in one year or less	3
Gross Unrealized Gains	3
Gross Unrealized Losses
Maturing in one year or less	(13)	(56)
Gross Unrealized Losses	(13)	(56)
Fair Value
Maturing in one year or less	72,068	94,264
Fair Value	$ 72,068	$ 94,264