Marketable Securities - (Details) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2020	Dec. 31, 2019
Investments
Amortized Cost	$ 267,374,000	$ 267,374,000	$ 300,552,000
Gross Unrealized Gain	1,353,000	1,353,000	118,000
Gross Unrealized Loss	(55,000)	(55,000)	(156,000)
Estimated Fair Value	268,672,000	268,672,000	300,514,000
Marketable securities other-than-temporarily impaired		0
Gross realized gains	3,000	70,000
U.S. treasury securities
Investments
Amortized Cost			4,991,000
Estimated Fair Value			4,991,000
Agency bonds
Investments
Amortized Cost	7,342,000	7,342,000	24,437,000
Gross Unrealized Gain	23,000	23,000	15,000
Gross Unrealized Loss			(1,000)
Estimated Fair Value	7,365,000	7,365,000	24,451,000
Corporate bonds
Investments
Amortized Cost	253,052,000	253,052,000	271,124,000
Gross Unrealized Gain	1,330,000	1,330,000	103,000
Gross Unrealized Loss	(55,000)	(55,000)	(155,000)
Estimated Fair Value	254,327,000	254,327,000	$ 271,072,000
Commercial Paper
Investments
Amortized Cost	6,980,000	6,980,000
Estimated Fair Value	$ 6,980,000	$ 6,980,000