Fair Value - Valuation of Financial Instruments by Pricing Observability Levels (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2025	Dec. 31, 2024
Assets, Fair Value Disclosure [Abstract]
Contingent consideration liability	$ 8,562	$ 9,385
Recurring
Assets, Fair Value Disclosure [Abstract]
Money markets	159,958	146,781
Securities segregated (U.S. Treasury Bills)	424,119	499,245
Derivatives	$ 14,014	$ 20,941
Derivative Asset, Statement of Financial Position [Extensible Enumeration]	Brokers and dealers	Brokers and dealers
Total assets measured at fair value and other measurements	$ 826,224	$ 877,384
Derivatives	$ 17,612	$ 14,944
Derivative Liability, Statement Of Financial Position, Extensible Enumeration, Not Disclosed Flag	Derivatives	Derivatives
Contingent consideration liability	$ 8,562	$ 9,385
Total liabilities measured at fair value	26,174	24,329
Recurring | Equity securities
Assets, Fair Value Disclosure [Abstract]
Equity securities	218,939	201,824
Recurring | Other investments
Assets, Fair Value Disclosure [Abstract]
Total investments	9,194	8,593
Level 1 | Recurring
Assets, Fair Value Disclosure [Abstract]
Money markets	159,958	146,781
Securities segregated (U.S. Treasury Bills)	0	0
Derivatives	75	2,835
Total assets measured at fair value	341,881	351,975
Derivatives	1,569	33
Contingent consideration liability	0	0
Total liabilities measured at fair value	1,569	33
Level 1 | Recurring | Equity securities
Assets, Fair Value Disclosure [Abstract]
Equity securities	172,654	193,766
Level 1 | Recurring | Other investments
Assets, Fair Value Disclosure [Abstract]
Other investments	9,194	8,593
Level 2 | Recurring
Assets, Fair Value Disclosure [Abstract]
Money markets	0	0
Securities segregated (U.S. Treasury Bills)	424,119	499,245
Derivatives	13,939	18,106
Total assets measured at fair value	481,943	523,272
Derivatives	16,043	14,911
Contingent consideration liability	0	0
Total liabilities measured at fair value	16,043	14,911
Level 2 | Recurring | Equity securities
Assets, Fair Value Disclosure [Abstract]
Equity securities	43,885	5,921
Level 2 | Recurring | Other investments
Assets, Fair Value Disclosure [Abstract]
Other investments	0	0
Level 3 | Recurring
Assets, Fair Value Disclosure [Abstract]
Money markets	0	0
Securities segregated (U.S. Treasury Bills)	0	0
Derivatives	0	0
Total assets measured at fair value	124	121
Derivatives	0	0
Contingent consideration liability	8,562	9,385
Total liabilities measured at fair value	8,562	9,385
Level 3 | Recurring | Equity securities
Assets, Fair Value Disclosure [Abstract]
Equity securities	124	121
Level 3 | Recurring | Other investments
Assets, Fair Value Disclosure [Abstract]
Other investments	0	0
NAV Expedient | Recurring
Assets, Fair Value Disclosure [Abstract]
Money markets	0	0
Securities segregated (U.S. Treasury Bills)	0	0
Derivatives	0	0
Total assets measured at fair value	2,276	2,016
Derivatives	0	0
Contingent consideration liability	0	0
Total liabilities measured at fair value	0	0
NAV Expedient | Recurring | Equity securities
Assets, Fair Value Disclosure [Abstract]
Equity securities	2,276	2,016
NAV Expedient | Recurring | Other investments
Assets, Fair Value Disclosure [Abstract]
Other investments	$ 0	$ 0