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	Sep. 30, 2019	Dec. 31, 2018
Level I [Member]
Financial Assets:
Cash and Cash Equivalents	$ 301,729	$ 759,849
Certificates of Deposit	0	0
Securities Purchased Under Agreements to Resell	0	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	0	0
Level II [Member]
Financial Assets:
Cash and Cash Equivalents	0	0
Certificates of Deposit	211,861	100,000
Securities Purchased Under Agreements to Resell	1,423	2,696
Receivables(1)	367,232	369,636
Contract Assets(2)	3,169	3,348
Receivable from Employees and Related Parties	20,655	23,836
Financial Liabilities:
Accounts Payable and Accrued Expenses	46,779	37,948
Securities Sold Under Agreements to Repurchase	25,062	25,075
Payable to Employees and Related Parties	28,748	31,894
Notes Payable	381,061	166,555
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	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	0	0
Carrying Amount [Member]
Financial Assets:
Cash and Cash Equivalents	301,729	759,849
Certificates of Deposit	211,861	100,000
Securities Purchased Under Agreements to Resell	1,423	2,696
Receivables(1)	370,248	370,023
Contract Assets(2)	3,362	3,374
Receivable from Employees and Related Parties	20,655	23,836
Financial Liabilities:
Accounts Payable and Accrued Expenses	46,779	37,948
Securities Sold Under Agreements to Repurchase	25,062	25,075
Payable to Employees and Related Parties	28,748	31,894
Notes Payable	372,526	168,612
Total [Member]
Financial Assets:
Cash and Cash Equivalents	301,729	759,849
Certificates of Deposit	211,861	100,000
Securities Purchased Under Agreements to Resell	1,423	2,696
Receivables(1)	367,232	369,636
Contract Assets(2)	3,169	3,348
Receivable from Employees and Related Parties	20,655	23,836
Financial Liabilities:
Accounts Payable and Accrued Expenses	46,779	37,948
Securities Sold Under Agreements to Repurchase	25,062	25,075
Payable to Employees and Related Parties	28,748	31,894
Notes Payable	381,061	166,555
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	8,851	9,717
Held-to-maturity Securities [Member] | Carrying Amount [Member]
Financial Assets:
Investments	8,851	9,717
Held-to-maturity Securities [Member] | Total [Member]
Financial Assets:
Investments	8,851	9,717
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	1,721	1,079
Equity Securities [Member] | Carrying Amount [Member]
Financial Assets:
Investments	1,721	1,079
Equity Securities [Member] | Total [Member]
Financial Assets:
Investments	$ 1,721	$ 1,079