Accounts Payable and Accrued and Other Liabilities - Accounts Payable and Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Payables and Accruals [Abstract]
Accounts payable	$ 38,588	$ 32,995
Payroll and employee benefits payable	189,557	165,449
Severance and retention bonus payable	26,476	24,281
Bonus payable	224,763	192,100
Commissions payable	130,306	142,499
Income tax payable	29,550	7,878
VAT payable	58,496	31,438
Current portion of operating lease liabilities	83,995	76,516
Current portion of interest rate swap contracts at fair value	34,886	0
Other accrued liabilities	135,814	115,640
Total accounts payable and accrued liabilities	$ 952,431	$ 788,796
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:AccountsPayableAndAccruedLiabilitiesCurrent	us-gaap:AccountsPayableAndAccruedLiabilitiesCurrent