Financial Instruments (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2019 USD ($) position item	Dec. 31, 2018 USD ($)
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Estimated Fair Value	$ 208,053	$ 107,461
Other than temporary impairment	$ 0
Number of investments sold | item	0
Number of investments, unrealized loss position | position	7
Amortized Cost
Less than one year	$ 97,180
Greater than one year but less than five years	110,077
Total	207,257
Fair Value
Less than one year	97,276
Greater than one year but less than five years	110,777
Total	208,053
Available-for-sale securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Cost	207,888	134,552
Unrealized Gain	895	36
Unrealized Loss	(100)	(588)
Estimated Fair Value	208,683	134,000
Money market deposits
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Cost	630	26,539
Estimated Fair Value	630	26,539
U.S. Treasury securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Cost	158,429	76,061
Unrealized Gain	733	29
Unrealized Loss	(65)	(405)
Estimated Fair Value	159,097	75,685
U.S. agency securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Cost	13,004	13,017
Unrealized Loss	(27)	(126)
Estimated Fair Value	12,977	12,891
Municipal securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Cost	35,825	18,935
Unrealized Gain	162	7
Unrealized Loss	(8)	(57)
Estimated Fair Value	35,979	18,885
Cash equivalents | Available-for-sale securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Estimated Fair Value	630	26,539
Short-term investments | Available-for-sale securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Estimated Fair Value	$ 208,053	$ 107,461