Fair Value Measurements - Marketable Securities (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2019
Marketable Securities
Other-than-temporary impairments associated with credit losses	$ 0	$ 0	$ 0
Interest earned on marketable securities	8,800,000	1,500,000
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest earned on marketable securities	8,800,000	$ 1,500,000
Cash and cash equivalents	$ 275,948,000			$ 189,052,000
Aggregate Fair Value | Reverse repurchase agreements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents				$ 35,800,000