Variable Interest Entities Special Purpose Entities, including Variable Interest Entities - Summary of Assets and Liabilities (Details) $ in Millions	Mar. 31, 2024 USD ($) Entity	Dec. 31, 2023 USD ($) Entity	Mar. 31, 2023 USD ($)
Debt Securities, Available-for-sale, Amortized Cost	$ 2,135	$ 2,197
Fixed maturity securities, at fair value	2,095
Restricted Cash	11	12	$ 10
Derivative assets	21	26
Other assets	129	129
Total assets	8,429	8,428	8,219
Interest Payable	487	475
Long-term debt	512	508
Derivative Liability	29	35
Other Liabilities	259	199
Liabilities	6,993	6,997
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	37	40
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	75	71
Variable Interest Entity, Primary Beneficiary [Member]
Fixed maturity securities, at fair value	2,162	2,167
Restricted Cash	252	246	$ 256
Financing Receivable, after Allowance for Credit Loss	1,604	1,663
Derivative assets	221	226
Other assets	92	92
Total assets	4,330	4,394
Long-term debt	2,671	2,710
Long-term Debt, at par less amortized discount	254	258
Long-term debt	2,925	2,967
Derivative Liability	1,170	1,197
Other Liabilities	6	5
Liabilities	4,340	4,404
Estimated Future Premium Payments Maximum Discounted Rate	239	235
Variable Interest Entity, Primary Beneficiary [Member] | Corporate Debt Securities [Member]
Fixed maturity securities, at fair value	2,070	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	7	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	2,070	2,072
Restricted Cash	251	245
Financing Receivable, after Allowance for Credit Loss	1,604	1,663
Derivative assets	221	226
Other assets	91	90
Total assets	4,236	4,296
Long-term debt	2,671	2,710
Long-term Debt, at par less amortized discount	98	99
Long-term debt	2,769	2,808
Derivative Liability	1,170	1,197
Cash collateral payable	239
Other Liabilities	6	4
Liabilities	4,184	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	2,070	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	155	159
Long-term debt	155	159
Derivative Liability	0	0
Other Liabilities	0	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