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, 2018	Dec. 31, 2017
Level I [Member]
Financial Assets:
Cash and Cash Equivalents	$ 273,984	$ 596,141
Certificates of Deposit		0
Debt Security Investment	0	0
Securities Purchased Under Agreements to Resell	0	0
Accounts Receivable	0	0
Receivable from Employees and Related Parties	0	0
Closely-held Equity Security	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	0
Level II [Member]
Financial Assets:
Cash and Cash Equivalents	0	0
Certificates of Deposit		63,527
Debt Security Investment	0	0
Securities Purchased Under Agreements to Resell	10,637	10,645
Accounts Receivable	223,106	184,993
Receivable from Employees and Related Parties	20,763	17,030
Closely-held Equity Security	0	0
Financial Liabilities:
Accounts Payable and Accrued Expenses	36,984	34,111
Securities Sold Under Agreements to Repurchase	32,476	30,027
Payable to Employees and Related Parties	44,807	31,167
Notes Payable	165,290	171,929
Subordinated Borrowings	100	6,859
Level III [Member]
Financial Assets:
Cash and Cash Equivalents	0	0
Certificates of Deposit		0
Debt Security Investment	11,132	10,995
Securities Purchased Under Agreements to Resell	0	0
Accounts Receivable	0	0
Receivable from Employees and Related Parties	0	0
Closely-held Equity Security	1,079	1,079
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	0
Carrying Amount [Member]
Financial Assets:
Cash and Cash Equivalents	273,984	596,141
Certificates of Deposit		63,527
Debt Security Investment	11,132	10,995
Securities Purchased Under Agreements to Resell	10,637	10,645
Accounts Receivable	223,106	184,993
Receivable from Employees and Related Parties	20,763	17,030
Closely-held Equity Security	1,079	1,079
Financial Liabilities:
Accounts Payable and Accrued Expenses	36,984	34,111
Securities Sold Under Agreements to Repurchase	32,476	30,027
Payable to Employees and Related Parties	44,807	31,167
Notes Payable	168,411	168,347
Subordinated Borrowings	99	6,799
Total [Member]
Financial Assets:
Cash and Cash Equivalents	273,984	596,141
Certificates of Deposit		63,527
Debt Security Investment	11,132	10,995
Securities Purchased Under Agreements to Resell	10,637	10,645
Accounts Receivable	223,106	184,993
Receivable from Employees and Related Parties	20,763	17,030
Closely-held Equity Security	1,079	1,079
Financial Liabilities:
Accounts Payable and Accrued Expenses	36,984	34,111
Securities Sold Under Agreements to Repurchase	32,476	30,027
Payable to Employees and Related Parties	44,807	31,167
Notes Payable	165,290	171,929
Subordinated Borrowings	$ 100	$ 6,859