Fair Value of Financial Instruments (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	$ 363,542,000	$ 226,708,000
Equity Method Investments	$ 3,600,000	$ 3,300,000
Equity Method Investment, Ownership Percentage	25.00%	27.70%
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	$ 10,065,000	$ 10,080,000
US Treasury and Government Short-term Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	39,086,000	32,630,000
Corporate notes, bonds and commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	314,391,000	183,998,000
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	363,542,000	226,708,000
Fair Value, Measurements, Recurring [Member] | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	10,065,000	10,080,000
Fair Value, Measurements, Recurring [Member] | US Treasury and Government Short-term Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	39,086,000	32,630,000
Fair Value, Measurements, Recurring [Member] | Corporate notes, bonds and commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	314,391,000	183,998,000
Fair Value, Measurements, Recurring [Member] | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	10,065,000	10,080,000
Fair Value, Measurements, Recurring [Member] | Level 1 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	10,065,000	10,080,000
Fair Value, Measurements, Recurring [Member] | Level 1 | US Treasury and Government Short-term Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0	0
Fair Value, Measurements, Recurring [Member] | Level 1 | Corporate notes, bonds and commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0	0
Fair Value, Measurements, Recurring [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	353,477,000	216,628,000
Fair Value, Measurements, Recurring [Member] | Level 2 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0	0
Fair Value, Measurements, Recurring [Member] | Level 2 | US Treasury and Government Short-term Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	39,086,000	32,630,000
Fair Value, Measurements, Recurring [Member] | Level 2 | Corporate notes, bonds and commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	314,391,000	183,998,000
Fair Value, Measurements, Recurring [Member] | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 | US Treasury and Government Short-term Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 | Corporate notes, bonds and commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	$ 0	$ 0