Fair Value of Financial Instruments (Details) - USD ($)	Jun. 30, 2019	Dec. 31, 2018
Financial assets subject to fair value measurements and the necessary disclosures
Equity Method Investment, Ownership Percentage	27.70%
Total available-for-sale securities	$ 301,043,000	$ 226,708,000
Equity Method Investments	4,100,000
Money market funds
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	38,437,000	10,080,000
US Treasury and Government Short-term Debt Securities [Member]
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	28,178,000	32,630,000
Corporate notes, bonds and commercial paper
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	234,428,000	183,998,000
Recurring basis
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	301,043,000	226,708,000
Recurring basis | Money market funds
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	38,437,000	10,080,000
Recurring basis | US Treasury and Government Short-term Debt Securities [Member]
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	28,178,000	32,630,000
Recurring basis | Corporate notes, bonds and commercial paper
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	234,428,000	183,998,000
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	38,437,000	10,080,000
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	38,437,000	10,080,000
Recurring basis | Quoted Market Prices in Active Markets (Level 1) | US Treasury and Government Short-term Debt Securities [Member]
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
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	262,606,000	216,628,000
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 Treasury and Government Short-term Debt Securities [Member]
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	28,178,000	32,630,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Corporate notes, bonds and commercial paper
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	234,428,000	183,998,000
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 Treasury and Government Short-term Debt Securities [Member]
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
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	$ 0	$ 0