Variable Interest Entities Special Purpose Entities, including Variable Interest Entities - Summary of Assets and Liabilities (Details) $ in Millions	Sep. 30, 2024 USD ($) Entity	Dec. 31, 2023 USD ($) Entity	Sep. 30, 2023 USD ($)
Debt Securities, Available-for-sale, Amortized Cost	$ 2,069	$ 2,197
Fixed maturity securities, at fair value	2,073
Restricted Cash	16	12	$ 13
Derivative assets	33	26
Other assets	228	129
Total assets	9,256	8,428	7,847
Interest Payable	515	475
Long-term debt	518	508
Derivative Liability	35	35
Other Liabilities	262	180
Liabilities	7,383	6,997
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	54	40
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	49	71
Variable Interest Entity, Primary Beneficiary [Member]
Fixed maturity securities, at fair value	2,238	2,167
Restricted Cash	47	246	$ 256
Financing Receivable, after Allowance for Credit Loss	1,651	1,663
Derivative assets	216	226
Other assets	110	92
Total assets	4,262	4,394
Long-term debt	2,738	2,710
Long-term Debt, at par less amortized discount	259	258
Long-term debt	2,996	2,967
Derivative Liability	1,223	1,197
Other Liabilities	10	5
Liabilities	4,271	4,404
Estimated Future Premium Payments Maximum Discounted Rate	41	235
Variable Interest Entity, Primary Beneficiary [Member] | Corporate Debt Securities [Member]
Fixed maturity securities, at fair value	2,146	2,072
Variable Interest Entity, Primary Beneficiary [Member] | Municipal Bonds [Member]
Debt Securities, Available-for-sale, Amortized Cost	85	88
Fixed maturity securities, at fair value	92	95
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	8	7
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	0
Variable Interest Entity, Primary Beneficiary [Member] | Ambac UK [Member]
Fixed maturity securities, at fair value	2,146	2,072
Restricted Cash	46	245
Financing Receivable, after Allowance for Credit Loss	1,651	1,663
Derivative assets	216	226
Other assets	109	90
Total assets	4,168	4,296
Long-term debt	2,738	2,710
Long-term Debt, at par less amortized discount	103	99
Long-term debt	2,841	2,808
Derivative Liability	1,223	1,197
Cash collateral payable	41
Other Liabilities	10	4
Liabilities	4,115	4,244
Estimated Future Premium Payments Maximum Discounted Rate		235
Variable Interest Entity, Primary Beneficiary [Member] | Ambac UK [Member] | Corporate Debt Securities [Member]
Fixed maturity securities, at fair value	2,146	2,072
Variable Interest Entity, Primary Beneficiary [Member] | Ambac UK [Member] | Municipal Bonds [Member]
Fixed maturity securities, at fair value	0	0
Variable Interest Entity, Primary Beneficiary [Member] | Ambac Assurance [Member]
Fixed maturity securities, at fair value	92	95
Restricted Cash	1	1
Financing Receivable, after Allowance for Credit Loss	0	0
Derivative assets	0	0
Other assets	1	2
Total assets	94	98
Long-term debt	0	0
Long-term Debt, at par less amortized discount	156	159
Long-term debt	156	159
Derivative Liability	0	0
Other Liabilities	1	1
Liabilities	156	160
Estimated Future Premium Payments Maximum Discounted Rate	0	0
Variable Interest Entity, Primary Beneficiary [Member] | Ambac Assurance [Member] | Corporate Debt Securities [Member]
Fixed maturity securities, at fair value	0	0
Variable Interest Entity, Primary Beneficiary [Member] | Ambac Assurance [Member] | Municipal Bonds [Member]
Fixed maturity securities, at fair value	$ 92	$ 95
Consolidated Entities [Member]
Number Of Consolidated Variable Interest Entities | Entity	6	6
Consolidated Entities [Member] | Ambac UK [Member]
Number Of Consolidated Variable Interest Entities | Entity	4	4
Consolidated Entities [Member] | Ambac Assurance [Member]
Number Of Consolidated Variable Interest Entities | Entity	2	2