Investments - Strategic Investment in Fortitude Re (Details) - USD ($)				1 Months Ended	3 Months Ended			9 Months Ended
	Jan. 01, 2024	Jun. 02, 2020	Nov. 13, 2018	May 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Mar. 31, 2020	Dec. 31, 2019
Schedule of Equity Method Investments [Line Items]
Investment					$ 1,699,400,000			$ 1,699,400,000			$ 2,443,600,000
Cumulative unrealized gains										$ 539,100,000	628,200,000
Investment income (loss)					584,100,000		$ 324,600,000	72,400,000	$ 1,565,100,000
Principal investment income (loss) from equity method investments (excluding performance allocations) - realized
Schedule of Equity Method Investments [Line Items]
Investment income (loss)					26,900,000		$ 30,900,000	105,300,000	$ 157,400,000
Fortitude Group Holdings, LLC
Schedule of Equity Method Investments [Line Items]
Ownership percentage acquired			19.90%
Payments to acquire equity method investments			$ 381,000,000
Deferred consideration, maximum			$ 95,000,000
Purchase price adjustment, maximum				$ 99,500,000
Purchase price adjustment, payable upon inability to make distribution				$ 79,600,000
Investment										$ 1,077,900,000	$ 1,200,900,000
Carlyle FRL
Schedule of Equity Method Investments [Line Items]
Investment					542,300,000			542,300,000
Reversal of investment income						$ 620,700,000
Investment, cost					465,400,000			465,400,000
Carlyle FRL | Principal investment income (loss) from equity method investments (excluding performance allocations) - realized
Schedule of Equity Method Investments [Line Items]
Investment income (loss)					$ 28,700,000			$ (703,900,000)
Fortitude Group Holdings, LLC | Fortitude Re
Schedule of Equity Method Investments [Line Items]
Ownership percentage			100.00%
Carlyle FRL | Fortitude Group Holdings, LLC
Schedule of Equity Method Investments [Line Items]
Ownership percentage acquired		51.60%
Ownership percentage		71.50%
T&D | Fortitude Group Holdings, LLC
Schedule of Equity Method Investments [Line Items]
Ownership percentage acquired		25.00%
Carlyle FRL and T&D | Fortitude Group Holdings, LLC
Schedule of Equity Method Investments [Line Items]
Ownership percentage		96.50%
Carlyle FRL | Fortitude Group Holdings, LLC
Schedule of Equity Method Investments [Line Items]
Ownership interest transferred		19.90%
AIG | Fortitude Group Holdings, LLC
Schedule of Equity Method Investments [Line Items]
Purchase price adjustment, adverse reserve developments, maximum		$ 500,000,000
Minimum | Fortitude Group Holdings, LLC | Fortitude Group Holdings, LLC
Schedule of Equity Method Investments [Line Items]
Assets commitment allocation time limit			30 months
Maximum | Fortitude Group Holdings, LLC | Fortitude Group Holdings, LLC
Schedule of Equity Method Investments [Line Items]
Assets commitment allocation time limit			36 months
Forecast | Fortitude Group Holdings, LLC
Schedule of Equity Method Investments [Line Items]
Purchase price adjustment, payable upon inability to make distribution	$ 19,900,000