Accounts Payable, Accrued Expenses and Other Liabilities - Components of Other Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Payables and Accruals [Abstract]
Accrued compensation	$ 309,710	$ 278,399
Payroll taxes payable	62,500	41,355
Contingent consideration	13,365	32,065
Credit enhancement deposit	0	25,000
Financial guarantee liability	32,652	15	$ 15
Other liabilities	$ 418,227	$ 376,834