Financial Instruments (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) position	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	$ 107,461	$ 106,247
Realized gain			$ 200
Other than temporary impairment	$ 0
Number of investments, unrealized loss position | position	16
Interest income	$ 1,800	1,200	$ 1,500
Amortized Cost
Less than one year	64,477
Greater than one year but less than five years	43,536
Total	108,013
Fair Value
Less than one year	64,276
Greater than one year but less than five years	43,185
Total	107,461
Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	134,552	110,490
Unrealized Gain	36	10
Unrealized Loss	(588)	(776)
Estimated Fair Value	134,000	109,724
Money market deposits
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	26,539	3,477
Estimated Fair Value	26,539	3,477
U.S. Treasury securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	76,061	67,480
Unrealized Gain	29	10
Unrealized Loss	(405)	(464)
Estimated Fair Value	75,685	67,026
U.S. agency securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	13,017	27,293
Unrealized Loss	(126)	(221)
Estimated Fair Value	12,891	27,072
Municipal securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	18,935	12,240
Unrealized Gain	7
Unrealized Loss	(57)	(91)
Estimated Fair Value	18,885	12,149
Cash equivalents | Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	26,539	3,477
Short-term investments | Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	$ 107,461	$ 106,247