Fair Value Measurements - Carrying Amount and Estimated Fair Value of Financial Instrument Assets and Liabilities which are Not Measured at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Level I [Member]
Financial Assets:
Cash and Cash Equivalents	$ 924,294	$ 630,818
Certificates of Deposit		0
Securities Purchased Under Agreements to Resell	0	0
Receivables(1)	0	0
Contract Assets(2)	0	0
Receivable from Employees and Related Parties	0	0
Financial Liabilities:
Accounts Payable and Accrued Expenses	0	0
Securities Sold Under Agreements to Repurchase	0	0
Payable to Employees and Related Parties	0	0
Notes Payable(3)	0	0
Level II [Member]
Financial Assets:
Cash and Cash Equivalents	0	0
Certificates of Deposit		214,796
Securities Purchased Under Agreements to Resell	8,766	13,566
Receivables(1)	342,398	357,047
Contract Assets(2)	21,517	33,854
Receivable from Employees and Related Parties	25,900	22,416
Financial Liabilities:
Accounts Payable and Accrued Expenses	41,084	39,726
Securities Sold Under Agreements to Repurchase	22,050	26,000
Payable to Employees and Related Parties	21,512	31,703
Notes Payable(3)	400,507	382,274
Level III [Member]
Financial Assets:
Cash and Cash Equivalents	0	0
Certificates of Deposit		0
Securities Purchased Under Agreements to Resell	0	0
Receivables(1)	0	0
Contract Assets(2)	0	0
Receivable from Employees and Related Parties	0	0
Financial Liabilities:
Accounts Payable and Accrued Expenses	0	0
Securities Sold Under Agreements to Repurchase	0	0
Payable to Employees and Related Parties	0	0
Notes Payable(3)	0	0
Carrying Amount [Member]
Financial Assets:
Cash and Cash Equivalents	924,294	630,818
Certificates of Deposit		214,796
Securities Purchased Under Agreements to Resell	8,766	13,566
Receivables(1)	345,331	359,909
Contract Assets(2)	21,697	34,029
Receivable from Employees and Related Parties	25,900	22,416
Financial Liabilities:
Accounts Payable and Accrued Expenses	41,084	39,726
Securities Sold Under Agreements to Repurchase	22,050	26,000
Payable to Employees and Related Parties	21,512	31,703
Notes Payable(3)	374,510	375,062
Total [Member]
Financial Assets:
Cash and Cash Equivalents	924,294	630,818
Certificates of Deposit		214,796
Securities Purchased Under Agreements to Resell	8,766	13,566
Receivables(1)	342,398	357,047
Contract Assets(2)	21,517	33,854
Receivable from Employees and Related Parties	25,900	22,416
Financial Liabilities:
Accounts Payable and Accrued Expenses	41,084	39,726
Securities Sold Under Agreements to Repurchase	22,050	26,000
Payable to Employees and Related Parties	21,512	31,703
Notes Payable(3)	400,507	382,274
Held-to-maturity Securities [Member] | Level I [Member]
Financial Assets:
Investments	0	0
Held-to-maturity Securities [Member] | Level II [Member]
Financial Assets:
Investments	0	0
Held-to-maturity Securities [Member] | Level III [Member]
Financial Assets:
Investments	6,684	9,235
Held-to-maturity Securities [Member] | Carrying Amount [Member]
Financial Assets:
Investments	6,684	9,235
Held-to-maturity Securities [Member] | Total [Member]
Financial Assets:
Investments	6,684	9,235
Equity Securities [Member] | Level I [Member]
Financial Assets:
Investments	0	0
Equity Securities [Member] | Level II [Member]
Financial Assets:
Investments	0	0
Equity Securities [Member] | Level III [Member]
Financial Assets:
Investments	1,724	1,772
Equity Securities [Member] | Carrying Amount [Member]
Financial Assets:
Investments	1,724	1,772
Equity Securities [Member] | Total [Member]
Financial Assets:
Investments	$ 1,724	$ 1,772