Variable Interest Entities Special Purpose Entities, including Variable Interest Entities - Summary of Assets and Liabilities (Details) $ in Millions	Mar. 31, 2021 USD ($) Entity	Dec. 31, 2020 USD ($) Entity	Mar. 31, 2020 USD ($)
Debt Securities, Available-for-sale, Amortized Cost	$ 2,739
Fixed maturity securities, at fair value	2,858
Restricted Cash	16	$ 13	$ 31
Derivative assets	74	93
Other assets	115	114
Total assets	12,840	13,220
Interest Payable	516	517
Long-term debt	2,661	2,739
Derivative Liability	86	114
Other Liabilities	123	106
Liabilities	11,697	12,074
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	136	149
Variable Interest Entity, Primary Beneficiary [Member]
Fixed maturity securities, at fair value	3,236	3,354
Restricted Cash	2	2	$ 2
Financing Receivable, after Allowance for Credit Loss	2,948	2,998
Derivative assets	38	41
Other assets	1	2
Total assets	6,225	6,398
Long-term debt	4,264	4,324
Long-term Debt, at par less amortized discount	163	169
Long-term debt	4,427	4,493
Derivative Liability	1,739	1,835
Liabilities	6,166	6,328
Variable Interest Entity, Primary Beneficiary [Member] | Corporate Debt Securities [Member]
Fixed maturity securities, at fair value	3,103	3,215
Variable Interest Entity, Primary Beneficiary [Member] | Municipal Bonds [Member]
Debt Securities, Available-for-sale, Amortized Cost	109	113
Fixed maturity securities, at fair value	133	139
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	23	27
Variable Interest Entity, Primary Beneficiary [Member] | Ambac UK [Member]
Fixed maturity securities, at fair value	3,103	3,215
Restricted Cash	1	1
Financing Receivable, after Allowance for Credit Loss	2,948	2,998
Derivative assets	38	41
Other assets	0	0
Total assets	6,090	6,255
Long-term debt	4,264	4,324
Long-term Debt, at par less amortized discount	0	0
Long-term debt	4,264	4,324
Derivative Liability	1,739	1,835
Liabilities	6,003	6,159
Variable Interest Entity, Primary Beneficiary [Member] | Ambac UK [Member] | Corporate Debt Securities [Member]
Fixed maturity securities, at fair value	3,103	3,215
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	133	139
Restricted Cash	1	1
Financing Receivable, after Allowance for Credit Loss	0	0
Derivative assets	0	0
Other assets	1	2
Total assets	135	143
Long-term debt	0	0
Long-term Debt, at par less amortized discount	163	169
Long-term debt	163	169
Derivative Liability	0	0
Liabilities	163	169
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	$ 133	$ 139
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	5	5
Consolidated Entities [Member] | Ambac Assurance [Member]
Number Of Consolidated Variable Interest Entities | Entity	1	1