Revenue and Related Matters - Schedule of Contract with Customer, Asset and Liability (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Assets:
Fees receivable, gross	$ 1,251,508	$ 1,334,012
Contract assets recorded in Prepaid expenses and other current assets	14,440	21,350
Contract liabilities:
Deferred revenues (current liability)	1,974,548	1,928,020
Non-current deferred revenues recorded in Other liabilities	26,754	24,409
Total contract liabilities	$ 2,001,302	$ 1,952,429