Financial Instruments (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2017 USD ($) position	Dec. 31, 2016 USD ($)
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	$ 106,692	$ 130,860
Number of investments, unrealized loss position | position	21
Interest income	$ 300
Amortized Cost
Less than one year	39,169
Greater than one year but less than five years	68,048
Total	107,217
Fair Value
Less than one year	39,100
Greater than one year but less than five years	67,592
Total	106,692
Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	107,737	142,362
Unrealized Gain	5
Unrealized Loss	(530)	(725)
Estimated Fair Value	107,212	141,637
Money market deposits
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	520	10,777
Estimated Fair Value	520	10,777
U.S. Treasury securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	40,751	57,846
Unrealized Loss	(346)	(404)
Estimated Fair Value	40,405	57,442
U.S. agency securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	50,160	56,261
Unrealized Loss	(152)	(147)
Estimated Fair Value	50,008	56,114
Municipal securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	16,306	17,478
Unrealized Gain	5
Unrealized Loss	(32)	(174)
Estimated Fair Value	16,279	17,304
Cash equivalents | Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	520	10,777
Short-term investments | Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	$ 106,692	$ 130,860