Marketable Securities - (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Investments
Amortized Cost	$ 293,734,000	$ 300,552,000
Gross Unrealized Gain	343,000	118,000
Gross Unrealized Loss	(886,000)	(156,000)
Estimated Fair Value	293,191,000	300,514,000
Marketable securities other-than-temporarily impaired	0
Gross realized gains	64,000
U.S. treasury securities
Investments
Amortized Cost		4,991,000
Estimated Fair Value		4,991,000
Agency bonds
Investments
Amortized Cost	35,643,000	24,437,000
Gross Unrealized Gain	157,000	15,000
Gross Unrealized Loss		(1,000)
Estimated Fair Value	35,800,000	24,451,000
Corporate bonds
Investments
Amortized Cost	258,091,000	271,124,000
Gross Unrealized Gain	186,000	103,000
Gross Unrealized Loss	(886,000)	(155,000)
Estimated Fair Value	$ 257,391,000	$ 271,072,000