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	Dec. 31, 2020	Dec. 31, 2019
Level 1 [Member]
Financial Assets:
Cash and Cash Equivalents	$ 822,598	$ 630,818
Certificates of Deposit		0
Securities Purchased Under Agreements to Resell		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
Payable to Employees and Related Parties	0	0
Notes Payable(3)	0	0
Level 2 [Member]
Financial Assets:
Cash and Cash Equivalents	0	0
Certificates of Deposit		214,796
Securities Purchased Under Agreements to Resell		13,566
Receivables(1)	434,083	357,047
Contract Assets(2)	34,052	33,854
Receivable from Employees and Related Parties	23,593	22,416
Financial Liabilities:
Accounts Payable and Accrued Expenses	37,961	39,726
Securities Sold Under Agreements to Repurchase		26,000
Payable to Employees and Related Parties	24,047	31,703
Notes Payable(3)	409,682	382,274
Level 3 [Member]
Financial Assets:
Cash and Cash Equivalents	0	0
Certificates of Deposit		0
Securities Purchased Under Agreements to Resell		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
Payable to Employees and Related Parties	0	0
Notes Payable(3)	0	0
Carrying Amount [Member]
Financial Assets:
Cash and Cash Equivalents	822,598	630,818
Certificates of Deposit		214,796
Securities Purchased Under Agreements to Resell		13,566
Receivables(1)	439,321	359,909
Contract Assets(2)	34,610	34,029
Receivable from Employees and Related Parties	23,593	22,416
Financial Liabilities:
Accounts Payable and Accrued Expenses	37,961	39,726
Securities Sold Under Agreements to Repurchase		26,000
Payable to Employees and Related Parties	24,047	31,703
Notes Payable(3)	376,492	375,062
Total [Member]
Financial Assets:
Cash and Cash Equivalents	822,598	630,818
Certificates of Deposit		214,796
Securities Purchased Under Agreements to Resell		13,566
Receivables(1)	434,083	357,047
Contract Assets(2)	34,052	33,854
Receivable from Employees and Related Parties	23,593	22,416
Financial Liabilities:
Accounts Payable and Accrued Expenses	37,961	39,726
Securities Sold Under Agreements to Repurchase		26,000
Payable to Employees and Related Parties	24,047	31,703
Notes Payable(3)	409,682	382,274
Held-to-maturity Securities [Member] | Level 1 [Member]
Financial Assets:
Investments	0	0
Held-to-maturity Securities [Member] | Level 2 [Member]
Financial Assets:
Investments	0	0
Held-to-maturity Securities [Member] | Level 3 [Member]
Financial Assets:
Investments	7,385	9,235
Held-to-maturity Securities [Member] | Carrying Amount [Member]
Financial Assets:
Investments	7,385	9,235
Held-to-maturity Securities [Member] | Total [Member]
Financial Assets:
Investments	7,385	9,235
Equity Securities [Member] | Level 1 [Member]
Financial Assets:
Investments	0	0
Equity Securities [Member] | Level 2 [Member]
Financial Assets:
Investments	0	0
Equity Securities [Member] | Level 3 [Member]
Financial Assets:
Investments	683	1,772
Equity Securities [Member] | Carrying Amount [Member]
Financial Assets:
Investments	683	1,772
Equity Securities [Member] | Total [Member]
Financial Assets:
Investments	$ 683	$ 1,772