Financial Instruments (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($) position	Dec. 31, 2017 USD ($)
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	$ 70,901	$ 106,247
Other than temporary impairment	$ 0
Number of investments, unrealized loss position | position	17
Amortized Cost
Less than one year	$ 33,114
Greater than one year but less than five years	38,645
Total	71,759
Fair Value
Less than one year	32,989
Greater than one year but less than five years	37,912
Total	70,901
Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	72,328	110,490
Unrealized Gain		10
Unrealized Loss	(858)	(776)
Estimated Fair Value	71,470	109,724
Money market deposits
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	569	3,477
Estimated Fair Value	569	3,477
U.S. Treasury securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	37,445	67,480
Unrealized Gain		10
Unrealized Loss	(580)	(464)
Estimated Fair Value	36,865	67,026
U.S. agency securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	22,544	27,293
Unrealized Loss	(212)	(221)
Estimated Fair Value	22,332	27,072
Municipal securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	11,770	12,240
Unrealized Loss	(66)	(91)
Estimated Fair Value	11,704	12,149
Cash equivalents | Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	569	3,477
Short-term investments | Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	$ 70,901	$ 106,247