Financial Instruments (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) position	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	$ 106,247	$ 130,860
Realized gain		200
Number of investments, unrealized loss position | position	19
Interest income	$ 1,200	1,500	$ 500
Amortized Cost
Less than one year	57,168
Greater than one year but less than five years	49,845
Total	107,013
Fair Value
Less than one year	57,046
Greater than one year but less than five years	49,201
Total	106,247
Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	110,490	142,362
Unrealized Gain	10
Unrealized Loss	(776)	(725)
Estimated Fair Value	109,724	141,637
Money market deposits
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	3,477	10,777
Estimated Fair Value	3,477	10,777
U.S. Treasury securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	67,480	57,846
Unrealized Gain	10
Unrealized Loss	(464)	(404)
Estimated Fair Value	67,026	57,442
U.S. agency securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	27,293	56,261
Unrealized Loss	(221)	(147)
Estimated Fair Value	27,072	56,114
Municipal securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	12,240	17,478
Unrealized Loss	(91)	(174)
Estimated Fair Value	12,149	17,304
Cash equivalents | Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	3,477	10,777
Short-term investments | Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	$ 106,247	$ 130,860