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, 2021	Dec. 31, 2020
Level 1 [Member]
Financial Assets:
Cash and Cash Equivalents	$ 436,287	$ 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	122,510
Receivables(1)	401,141	434,083
Contract Assets(2)	60,799	34,052
Receivable from Employees and Related Parties	19,043	23,593
Financial Liabilities:
Accounts Payable and Accrued Expenses	43,776	37,961
Payable to Employees and Related Parties	48,097	24,047
Notes Payable	396,393	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	436,287	822,598
Certificates of Deposit, at Carrying Value	122,510
Receivables(1)	404,529	439,321
Contract Assets(2)	61,428	34,610
Receivable from Employees and Related Parties	19,043	23,593
Financial Liabilities:
Accounts Payable and Accrued Expenses	43,776	37,961
Payable to Employees and Related Parties	48,097	24,047
Notes Payable	376,778	376,492
Total [Member]
Financial Assets:
Cash and Cash Equivalents	436,287	822,598
Certificates of Deposit, at Carrying Value	122,510
Receivables(1)	401,141	434,083
Contract Assets(2)	60,799	34,052
Receivable from Employees and Related Parties	19,043	23,593
Financial Liabilities:
Accounts Payable and Accrued Expenses	43,776	37,961
Payable to Employees and Related Parties	48,097	24,047
Notes Payable	396,393	409,682
Held-to-maturity Securities [Member] | Level 1 [Member]
Financial Assets:
Investments		0
Held-to-maturity Securities [Member] | Level 2 [Member]
Financial Assets:
Investments		0
Held-to-maturity Securities [Member] | Level 3 [Member]
Financial Assets:
Investments		7,385
Held-to-maturity Securities [Member] | Carrying Amount [Member]
Financial Assets:
Investments		7,385
Held-to-maturity Securities [Member] | Total [Member]
Financial Assets:
Investments		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	691	683
Equity Securities [Member] | Carrying Amount [Member]
Financial Assets:
Investments	691	683
Equity Securities [Member] | Total [Member]
Financial Assets:
Investments	$ 691	$ 683