Marketable Securities - (Details) - USD ($)	3 Months Ended	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Investments
Amortized Cost	$ 282,350,000	$ 282,350,000	$ 300,552,000
Gross Unrealized Gain	1,941,000	1,941,000	118,000
Gross Unrealized Loss	(6,000)	(6,000)	(156,000)
Estimated Fair Value	284,285,000	284,285,000	300,514,000
Marketable securities other-than-temporarily impaired		0
Gross realized gains	4,000	68,000
U.S. treasury securities
Investments
Amortized Cost			4,991,000
Estimated Fair Value			4,991,000
Agency bonds
Investments
Amortized Cost	24,366,000	24,366,000	24,437,000
Gross Unrealized Gain	83,000	83,000	15,000
Gross Unrealized Loss			(1,000)
Estimated Fair Value	24,449,000	24,449,000	24,451,000
Corporate bonds
Investments
Amortized Cost	251,022,000	251,022,000	271,124,000
Gross Unrealized Gain	1,858,000	1,858,000	103,000
Gross Unrealized Loss	(6,000)	(6,000)	(155,000)
Estimated Fair Value	252,874,000	252,874,000	$ 271,072,000
Commercial Paper
Investments
Amortized Cost	6,962,000	6,962,000
Estimated Fair Value	$ 6,962,000	$ 6,962,000