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	Jun. 30, 2018	Dec. 31, 2017
Level I [Member]
Financial Assets:
Cash and Cash Equivalents	$ 370,576	$ 596,141
Certificates of Deposit	0	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
Level II [Member]
Financial Assets:
Cash and Cash Equivalents	0	0
Certificates of Deposit	50,000	63,527
Debt Security Investment	0	0
Securities Purchased Under Agreements to Resell	10,625	10,645
Accounts Receivable	273,997	184,993
Receivable from Employees and Related Parties	21,099	17,030
Closely-held Equity Security	0	0
Financial Liabilities:
Accounts Payable and Accrued Expenses	39,737	34,111
Securities Sold Under Agreements to Repurchase	29,402	30,027
Payable to Employees and Related Parties	44,581	31,167
Notes Payable	166,037	171,929
Subordinated Borrowings		6,859
Level III [Member]
Financial Assets:
Cash and Cash Equivalents	0	0
Certificates of Deposit	0	0
Debt Security Investment	9,592	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
Carrying Amount [Member]
Financial Assets:
Cash and Cash Equivalents	370,576	596,141
Certificates of Deposit	50,000	63,527
Debt Security Investment	9,592	10,995
Securities Purchased Under Agreements to Resell	10,625	10,645
Accounts Receivable	273,997	184,993
Receivable from Employees and Related Parties	21,099	17,030
Closely-held Equity Security	1,079	1,079
Financial Liabilities:
Accounts Payable and Accrued Expenses	39,737	34,111
Securities Sold Under Agreements to Repurchase	29,402	30,027
Payable to Employees and Related Parties	44,581	31,167
Notes Payable	168,477	168,347
Subordinated Borrowings		6,799
Total [Member]
Financial Assets:
Cash and Cash Equivalents	370,576	596,141
Certificates of Deposit	50,000	63,527
Debt Security Investment	9,592	10,995
Securities Purchased Under Agreements to Resell	10,625	10,645
Accounts Receivable	273,997	184,993
Receivable from Employees and Related Parties	21,099	17,030
Closely-held Equity Security	1,079	1,079
Financial Liabilities:
Accounts Payable and Accrued Expenses	39,737	34,111
Securities Sold Under Agreements to Repurchase	29,402	30,027
Payable to Employees and Related Parties	44,581	31,167
Notes Payable	$ 166,037	171,929
Subordinated Borrowings		$ 6,859