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, 2021	Dec. 31, 2020
Level 1 [Member]
Financial Assets:
Cash and Cash Equivalents	$ 403,948	$ 822,598
Certificates of Deposit, at Carrying Value	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
Payable to Employees and Related Parties	0	0
Notes Payable	0	0
Level 2 [Member]
Financial Assets:
Cash and Cash Equivalents	0	0
Certificates of Deposit, at Carrying Value	73,877
Receivables(1)	421,717	434,083
Contract Assets(2)	31,479	34,052
Receivable from Employees and Related Parties	23,074	23,593
Financial Liabilities:
Accounts Payable and Accrued Expenses	40,770	37,961
Payable to Employees and Related Parties	47,042	24,047
Notes Payable	388,391	409,682
Level 3 [Member]
Financial Assets:
Cash and Cash Equivalents	0	0
Certificates of Deposit, at Carrying Value	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
Payable to Employees and Related Parties	0	0
Notes Payable	0	0
Carrying Amount [Member]
Financial Assets:
Cash and Cash Equivalents	403,948	822,598
Certificates of Deposit, at Carrying Value	73,877
Receivables(1)	424,971	439,321
Contract Assets(2)	31,972	34,610
Receivable from Employees and Related Parties	23,074	23,593
Financial Liabilities:
Accounts Payable and Accrued Expenses	40,770	37,961
Payable to Employees and Related Parties	47,042	24,047
Notes Payable	376,491	376,492
Total [Member]
Financial Assets:
Cash and Cash Equivalents	403,948	822,598
Certificates of Deposit, at Carrying Value	73,877
Receivables(1)	421,717	434,083
Contract Assets(2)	31,479	34,052
Receivable from Employees and Related Parties	23,074	23,593
Financial Liabilities:
Accounts Payable and Accrued Expenses	40,770	37,961
Payable to Employees and Related Parties	47,042	24,047
Notes Payable	388,391	409,682
Held-to-maturity Securities [Member] | Level 1 [Member]
Financial Assets:
Investments	0	0
Held-to-maturity Securities [Member] | Level 2 [Member]
Financial Assets:
Investments	0	0
Held-to-maturity Securities [Member] | Level 3 [Member]
Financial Assets:
Investments	6,686	7,385
Held-to-maturity Securities [Member] | Carrying Amount [Member]
Financial Assets:
Investments	6,686	7,385
Held-to-maturity Securities [Member] | Total [Member]
Financial Assets:
Investments	6,686	7,385
Equity Securities [Member] | Level 1 [Member]
Financial Assets:
Investments	0	0
Equity Securities [Member] | Level 2 [Member]
Financial Assets:
Investments	0	0
Equity Securities [Member] | Level 3 [Member]
Financial Assets:
Investments	688	683
Equity Securities [Member] | Carrying Amount [Member]
Financial Assets:
Investments	688	683
Equity Securities [Member] | Total [Member]
Financial Assets:
Investments	$ 688	$ 683