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Investments - Narrative (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended	9 Months Ended
	Jul. 01, 2023	Sep. 30, 2024	Sep. 30, 2023	Jun. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023	Dec. 31, 2022
Net Investment Income [Line Items]
Management fees		$ 12,207	$ 10,958		$ 36,640	$ 35,806
Restricted cash and cash equivalents		93,883			93,883		$ 106,351
Cash, cash equivalents and restricted cash		1,051,255	903,527		1,051,255	903,527	900,860	$ 1,207,203
Cash and cash equivalents		957,372			957,372		794,509
Capital Requirements | Asset Pledged as Collateral
Net Investment Income [Line Items]
Investments, fair value		250,300			250,300		232,200
Restricted cash and cash equivalents		2,700			2,700		7,200
Trust Accounts | Asset Pledged as Collateral
Net Investment Income [Line Items]
Investments, fair value		89,100			89,100		54,100
Restricted cash and cash equivalents		1,600			1,600		1,500
Underwriting Obligations | Asset Pledged as Collateral
Net Investment Income [Line Items]
Investments, fair value		32,500			32,500		37,200
Restricted cash and cash equivalents		89,600			89,600		97,400
Escrow Funds | Asset Pledged as Collateral
Net Investment Income [Line Items]
Restricted cash and cash equivalents		0			0		300
TS Hamilton Fund
Net Investment Income [Line Items]
Management fee percentage	2.50%			3.00%
Management fees		11,800	10,400		35,000	34,400
Incentive allocation	30.00%
Incentive allocation, reduction (as a percent)	50.00%
Incentive allocation, cumulative net profits (as a percent)	200.00%
Incentive allocation, excess profits (as a percent)	25.00%			20.00%
Incentive allocation, net profits (as a percent)	10.00%			15.00%
Incentive allocation expense (income)		(17,200)	$ 9,100		172,200	$ 15,100
Cash and cash equivalents		$ 560,757			$ 560,757		$ 479,255
TS Hamilton Fund | Hamilton Re
Net Investment Income [Line Items]
Initial term		3 years			3 years
Capital account withdrawal, monthly liquidity, number of calendar days prior to month end		15 days			15 days
Capital account withdrawal, daily liquidity, number of business day's written notice		1 day			1 day
Two Sigma Futures Portfolio, LLC (FTV)
Net Investment Income [Line Items]
Percentage of ownership in funds		18.50%			18.50%
Two Sigma Spectrum Portfolio, LLC (STV)
Net Investment Income [Line Items]
Percentage of ownership in funds		17.90%			17.90%
Two Sigma Equity Spectrum Portfolio, LLC (ESTV)
Net Investment Income [Line Items]
Percentage of ownership in funds		9.80%			9.80%
Two Sigma Hamilton Fund, LLC | Hamilton Re
Net Investment Income [Line Items]
Commitment for investment in Two Sigma Funds	$ 1,800,000
Investment in Two Sigma Funds, as a percentage of net tangible assets	60.00%			95.00%

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