Financial Instruments (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) position	Dec. 31, 2017 USD ($)
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	$ 80,023	$ 106,247
Other than temporary impairment	$ 0
Number of investments, unrealized loss position | position	17
Amortized Cost
Less than one year	$ 39,157
Greater than one year but less than five years	41,770
Total	80,927
Fair Value
Less than one year	39,021
Greater than one year but less than five years	41,002
Total	80,023
Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	102,276	110,490
Unrealized Gain	1	10
Unrealized Loss	(905)	(776)
Estimated Fair Value	101,372	109,724
Money market deposits
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	21,349	3,477
Estimated Fair Value	21,349	3,477
U.S. Treasury securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	46,496	67,480
Unrealized Gain	1	10
Unrealized Loss	(577)	(464)
Estimated Fair Value	45,920	67,026
U.S. agency securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	22,562	27,293
Unrealized Loss	(233)	(221)
Estimated Fair Value	22,329	27,072
Municipal securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	11,869	12,240
Unrealized Loss	(95)	(91)
Estimated Fair Value	11,774	12,149
Cash equivalents | Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	21,349	3,477
Short-term investments | Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	$ 80,023	$ 106,247