Fair Value of Financial Instruments (Details) - USD ($)	Jun. 30, 2020	Dec. 31, 2019
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	$ 441,422,000	$ 363,542,000
Equity method investment, ownership percentage	25.00%
Equity method investment	$ 3,300,000
Money market funds
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	50,414,000	10,065,000
US Government bonds and notes
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	145,236,000	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	245,772,000	314,391,000
Recurring basis
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	441,422,000	363,542,000
Recurring basis | Money market funds
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	50,414,000	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	145,236,000	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	245,772,000	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	50,414,000	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	50,414,000	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	391,008,000	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	145,236,000	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	245,772,000	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