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, 2018	Dec. 31, 2017
Level I [Member]
Financial Assets:
Cash and Cash Equivalents	$ 759,849	$ 596,141
Certificates of Deposit	0	0
Securities Purchased Under Agreements to Resell	0	0
Receivables(1)	0
Contract Assets(2)	0
Accounts Receivable		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	0	0
Subordinated Borrowings		0
Level II [Member]
Financial Assets:
Cash and Cash Equivalents	0	0
Certificates of Deposit	100,000	63,527
Securities Purchased Under Agreements to Resell	2,696	10,645
Receivables(1)	369,636
Contract Assets(2)	3,348
Accounts Receivable		184,993
Receivable from Employees and Related Parties	23,836	17,030
Financial Liabilities:
Accounts Payable and Accrued Expenses	37,948	34,111
Securities Sold Under Agreements to Repurchase	25,075	30,027
Payable to Employees and Related Parties	31,894	31,167
Notes Payable	166,555	171,929
Subordinated Borrowings		6,859
Level III [Member]
Financial Assets:
Cash and Cash Equivalents	0	0
Certificates of Deposit	0	0
Securities Purchased Under Agreements to Resell	0	0
Receivables(1)	0
Contract Assets(2)	0
Accounts Receivable		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	0	0
Subordinated Borrowings		0
Carrying Amount [Member]
Financial Assets:
Cash and Cash Equivalents	759,849	596,141
Certificates of Deposit	100,000	63,527
Securities Purchased Under Agreements to Resell	2,696	10,645
Receivables(1)	370,023
Contract Assets(2)	3,374
Accounts Receivable		184,993
Receivable from Employees and Related Parties	23,836	17,030
Financial Liabilities:
Accounts Payable and Accrued Expenses	37,948	34,111
Securities Sold Under Agreements to Repurchase	25,075	30,027
Payable to Employees and Related Parties	31,894	31,167
Notes Payable	168,612	168,347
Subordinated Borrowings		6,799
Total [Member]
Financial Assets:
Cash and Cash Equivalents	759,849	596,141
Certificates of Deposit	100,000	63,527
Securities Purchased Under Agreements to Resell	2,696	10,645
Receivables(1)	369,636
Contract Assets(2)	3,348
Accounts Receivable		184,993
Receivable from Employees and Related Parties	23,836	17,030
Financial Liabilities:
Accounts Payable and Accrued Expenses	37,948	34,111
Securities Sold Under Agreements to Repurchase	25,075	30,027
Payable to Employees and Related Parties	31,894	31,167
Notes Payable	166,555	171,929
Subordinated Borrowings		6,859
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	9,717	10,995
Held-to-maturity Securities [Member] | Carrying Amount [Member]
Financial Assets:
Investments	9,717	10,995
Held-to-maturity Securities [Member] | Total [Member]
Financial Assets:
Investments	9,717	10,995
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	1,079	1,079
Equity Securities [Member] | Carrying Amount [Member]
Financial Assets:
Investments	1,079	1,079
Equity Securities [Member] | Total [Member]
Financial Assets:
Investments	$ 1,079	$ 1,079