Marketable Securities - (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Investments
Amortized Cost	$ 300,552,000
Gross Unrealized Gain	118,000
Gross Unrealized Loss	(156,000)
Estimated Fair Value	300,514,000	$ 0
Unrealized losses in a continuous loss position for more than twelve consecutive months	0
Unrealized losses in a continuous loss position for less than twelve consecutive months	156,000
Gross realized gains	0
Gross realized losses	0
Interest income	2,000,000	$ 0
Cost Maturities:
Within 1 year	168,186,000
After 1 through 5	132,366,000
Total investments available for sale	300,552,000
Fair Value Maturities:
Within 1 year	168,204,000
After 1 through 5	132,310,000
Total investments available for sale	300,514,000
U.S. treasury securities
Investments
Amortized Cost	4,991,000
Estimated Fair Value	4,991,000
Agency bonds
Investments
Amortized Cost	24,437,000
Gross Unrealized Gain	15,000
Gross Unrealized Loss	(1,000)
Estimated Fair Value	24,451,000
Corporate bonds
Investments
Amortized Cost	271,124,000
Gross Unrealized Gain	103,000
Gross Unrealized Loss	(155,000)
Estimated Fair Value	$ 271,072,000