Fair Value Measurements for Operating Entities and Consolidated Funds - Carrying Value Disclosures (Details) - USD ($)		Sep. 30, 2021	Dec. 31, 2020		Sep. 30, 2020
Cash and cash equivalents		$ 1,057,123,000	$ 645,169,000		$ 539,733,000
Cash collateral pledged		48,996,000	110,743,000		110,666,000
Segregated Cash		177,693,000	185,141,000		139,152,000
Securities Purchased under Agreements to Resell		0	191,000
Securities borrowed		2,169,056,000	1,908,187,000
Loans Receivable, Net		3,479,000	7,682,000
Cash and cash equivalents, Consolidated Funds		426,000	417,000		$ 946,000
Securities Sold under Agreements to Repurchase		53,839,000	5,036,000
Securities sold under agreements to repurchase, Fair value		56,121,000	5,544,000
Securities Loaned		3,352,456,000	2,476,414,000
Convertible debt		0	80,808,000	[1]
Long-term Debt	[2]	479,503,000	383,067,000
Convertible Debt
Unamortized discount			6,700,000
Notes Payable, Other Payables
Unamortized discount		1,400,000
Debt Instrument, Unamortized Premium		300,000	400,000
Level 1
Cash and cash equivalents, Fair Value		1,057,123,000	645,169,000
Segregated cash fair value disclosures		177,693,000	185,141,000
Cash and cash equivalents, Consolidated Funds, Fair Value		426,000	417,000
Level 2
Cash collateral pledged, Fair Value		48,996,000	110,743,000
Federal Funds Sold and Securities Borrowed or Purchased under Agreements to Resell, Fair Value Disclosure		0	204,000
Securities Borrowed, Fair Value Disclosure		2,169,056,000	1,908,187,000
Securities sold under agreements to repurchase, Fair value		56,121,000	5,544,000
Securities loaned, fair value disclosure		3,352,456,000	2,476,414,000
Convertible debt, Fair Value		0	135,444,000	[3]
Notes payable and other debt, Fair Value	[4]	515,708,000	405,840,000
Level 3
Loans Receivable, Fair Value Disclosure	[5]	$ 3,479,000	$ 7,682,000

[1] The carrying amount of the convertible debt includes an unamortized discount of $6.7 million as of December 31, 2020.

[2] The carrying amount of the notes payable and other debt includes an unamortized discount and unamortized premium of $1.4 million and $0.3 million as of September 30, 2021, respectively and unamortized premium of $0.4 million as of December 31, 2020.

[3] The convertible debt includes the conversion option and is based on the last broker quote available.

[4] Notes payable and other debt are based on the last broker quote available.

[5] The fair market value of level 3 loans is calculated using discounted cash flows where applicable.