Financial Instruments (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2019 USD ($) position item	Dec. 31, 2018 USD ($)
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Estimated Fair Value	$ 186,700	$ 107,461
Other than temporary impairment	$ 0
Number of investments sold | item	0
Number of investments, unrealized loss position | position	9
Amortized Cost
Less than one year	$ 86,161
Greater than one year but less than five years	99,680
Total	185,841
Fair Value
Less than one year	86,343
Greater than one year but less than five years	100,357
Total	186,700
Available-for-sale securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Cost	191,507	134,552
Unrealized Gain	955	36
Unrealized Loss	(96)	(588)
Estimated Fair Value	192,366	134,000
Money market deposits
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Cost	5,666	26,539
Estimated Fair Value	5,666	26,539
U.S. Treasury securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Cost	130,913	76,061
Unrealized Gain	794	29
Unrealized Loss	(72)	(405)
Estimated Fair Value	131,635	75,685
U.S. agency securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Cost	7,000	13,017
Unrealized Loss		(126)
Estimated Fair Value	7,000	12,891
Municipal securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Cost	47,928	18,935
Unrealized Gain	161	7
Unrealized Loss	(24)	(57)
Estimated Fair Value	48,065	18,885
Cash equivalents | Available-for-sale securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Estimated Fair Value	5,666	26,539
Short-term investments | Available-for-sale securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Estimated Fair Value	$ 186,700	$ 107,461