Summary of Significant Accounting Policies - Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	$ 12,772	$ 9,261
Letter of Credit
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	5,100	$ 9,300
Initial Capitalization
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	1,500
Insurance Premiums
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	$ 6,100