Fair Value Measurements - Marketable Securities (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Marketable Securities
Other-than-temporary impairments associated with credit losses	$ 0	$ 0	$ 0
Interest earned on marketable securities	20,800,000	3,000,000.0	$ 2,600,000
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	189,052,000	465,217,000
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	$ 35,000,000.0