Financial Instruments (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($) position	Dec. 31, 2017 USD ($)
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	$ 130,891	$ 106,247
Other than temporary impairment	$ 0
Number of investments, unrealized loss position | position	22
Amortized Cost
Less than one year	$ 89,375
Greater than one year but less than five years	42,410
Total	131,785
Fair Value
Less than one year	89,188
Greater than one year but less than five years	41,703
Total	130,891
Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	149,665	110,490
Unrealized Gain		10
Unrealized Loss	(894)	(776)
Estimated Fair Value	148,771	109,724
Money market deposits
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	17,880	3,477
Estimated Fair Value	17,880	3,477
U.S. Treasury securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	101,087	67,480
Unrealized Gain		10
Unrealized Loss	(619)	(464)
Estimated Fair Value	100,468	67,026
U.S. agency securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	19,029	27,293
Unrealized Loss	(180)	(221)
Estimated Fair Value	18,849	27,072
Municipal securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	11,669	12,240
Unrealized Loss	(95)	(91)
Estimated Fair Value	11,574	12,149
Cash equivalents | Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	17,880	3,477
Short-term investments | Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	$ 130,891	$ 106,247