Fair Value of Financial Instruments (Details) - USD ($)	Sep. 30, 2020	Dec. 31, 2019
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	$ 461,369,000	$ 363,542,000
Equity method investment, ownership percentage	25.00%
Equity method investment	$ 3,100,000
Money market funds
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	25,123,000	10,065,000
US Government bonds and notes
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	203,461,000	39,086,000
Corporate notes, bonds and commercial paper and other
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	232,785,000	314,391,000
Recurring basis
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	461,369,000	363,542,000
Recurring basis | Money market funds
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	25,123,000	10,065,000
Recurring basis | US Government bonds and notes
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	203,461,000	39,086,000
Recurring basis | Corporate notes, bonds and commercial paper and other
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	232,785,000	314,391,000
Recurring basis | Quoted market prices in active markets (Level 1)
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	25,123,000	10,065,000
Recurring basis | Quoted market prices in active markets (Level 1) | Money market funds
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	25,123,000	10,065,000
Recurring basis | Quoted market prices in active markets (Level 1) | US Government bonds and notes
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	0	0
Recurring basis | Quoted market prices in active markets (Level 1) | Corporate notes, bonds and commercial paper and other
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	0	0
Recurring basis | Significant other observable inputs (Level 2)
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	436,246,000	353,477,000
Recurring basis | Significant other observable inputs (Level 2) | Money market funds
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	0	0
Recurring basis | Significant other observable inputs (Level 2) | US Government bonds and notes
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	203,461,000	39,086,000
Recurring basis | Significant other observable inputs (Level 2) | Corporate notes, bonds and commercial paper and other
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	232,785,000	314,391,000
Recurring basis | Significant unobservable inputs (Level 3)
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	0	0
Recurring basis | Significant unobservable inputs (Level 3) | Money market funds
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	0	0
Recurring basis | Significant unobservable inputs (Level 3) | US Government bonds and notes
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	0	0
Recurring basis | Significant unobservable inputs (Level 3) | Corporate notes, bonds and commercial paper and other
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	$ 0	$ 0