Financial Instruments (Details) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2017 USD ($) position	Jun. 30, 2017 USD ($) position	Dec. 31, 2016 USD ($)
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	$ 94,988	$ 94,988	$ 130,860
Other than temporary impairment		$ 0
Number of investments, unrealized loss position | position	19	19
Interest income	$ 200	$ 500
Amortized Cost
Less than one year	31,831	31,831
Greater than one year but less than five years	63,620	63,620
Total	95,451	95,451
Fair Value
Less than one year	31,776	31,776
Greater than one year but less than five years	63,212	63,212
Total	94,988	94,988
Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	95,451	95,451	142,362
Unrealized Gain	6	6
Unrealized Loss	(469)	(469)	(725)
Estimated Fair Value	94,988	94,988	141,637
Money market deposits
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost			10,777
Estimated Fair Value			10,777
U.S. Treasury securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	35,387	35,387	57,846
Unrealized Loss	(303)	(303)	(404)
Estimated Fair Value	35,084	35,084	57,442
U.S. agency securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	47,130	47,130	56,261
Unrealized Loss	(151)	(151)	(147)
Estimated Fair Value	46,979	46,979	56,114
Municipal securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	12,934	12,934	17,478
Unrealized Gain	6	6
Unrealized Loss	(15)	(15)	(174)
Estimated Fair Value	12,925	12,925	17,304
Cash equivalents | Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value			10,777
Short-term investments | Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	$ 94,988	$ 94,988	$ 130,860