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	Mar. 31, 2020	Dec. 31, 2019
Level I [Member]
Financial Assets:
Cash and Cash Equivalents	$ 575,513	$ 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	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		13,566
Receivables(1)	303,968	357,047
Contract Assets(2)	40,390	33,854
Receivable from Employees and Related Parties	24,697	22,416
Financial Liabilities:
Accounts Payable and Accrued Expenses	32,712	39,726
Securities Sold Under Agreements to Repurchase	20,422	26,000
Payable to Employees and Related Parties	35,858	31,703
Notes Payable(3)	372,027	382,274
Level III [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	0
Payable to Employees and Related Parties	0	0
Notes Payable(3)	0	0
Carrying Amount [Member]
Financial Assets:
Cash and Cash Equivalents	575,513	630,818
Certificates of Deposit		214,796
Securities Purchased Under Agreements to Resell		13,566
Receivables(1)	307,031	359,909
Contract Assets(2)	41,155	34,029
Receivable from Employees and Related Parties	24,697	22,416
Financial Liabilities:
Accounts Payable and Accrued Expenses	32,712	39,726
Securities Sold Under Agreements to Repurchase	20,422	26,000
Payable to Employees and Related Parties	35,858	31,703
Notes Payable(3)	373,070	375,062
Total [Member]
Financial Assets:
Cash and Cash Equivalents	575,513	630,818
Certificates of Deposit		214,796
Securities Purchased Under Agreements to Resell		13,566
Receivables(1)	303,968	357,047
Contract Assets(2)	40,390	33,854
Receivable from Employees and Related Parties	24,697	22,416
Financial Liabilities:
Accounts Payable and Accrued Expenses	32,712	39,726
Securities Sold Under Agreements to Repurchase	20,422	26,000
Payable to Employees and Related Parties	35,858	31,703
Notes Payable(3)	372,027	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	7,230	9,235
Held-to-maturity Securities [Member] | Carrying Amount [Member]
Financial Assets:
Investments	7,230	9,235
Held-to-maturity Securities [Member] | Total [Member]
Financial Assets:
Investments	7,230	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	1,728	1,772
Equity Securities [Member] | Carrying Amount [Member]
Financial Assets:
Investments	1,728	1,772
Equity Securities [Member] | Total [Member]
Financial Assets:
Investments	$ 1,728	$ 1,772