Fair Value of Financial Instruments (Details) - USD ($)	Mar. 31, 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	$ 264,477,000	$ 226,708,000
Equity Method Investments	3,200,000.0
Money market funds
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	10,849,000	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	41,479,000	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	212,149,000	183,998,000
Recurring basis
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	264,477,000	226,708,000
Recurring basis | Money market funds
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	10,849,000	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	41,479,000	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	212,149,000	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	10,849,000	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	10,849,000	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	253,628,000	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	41,479,000	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	212,149,000	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