Variable Interest Entities Special Purpose Entities, including Variable Interest Entities - Summary of Assets and Liabilities (Details) $ in Millions	Jun. 30, 2021 USD ($) Entity	Dec. 31, 2020 USD ($) Entity	Jun. 30, 2020 USD ($)
Debt Securities, Available-for-sale, Amortized Cost	$ 2,651
Fixed maturity securities, at fair value	2,789
Restricted Cash	17	$ 13	$ 15
Derivative assets	82	93
Other assets	121	114
Total assets	12,907	13,220
Interest Payable	532	517
Long-term debt	2,671	2,739
Derivative Liability	98	114
Other Liabilities	102	106
Liabilities	11,764	12,074
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	147	149
Variable Interest Entity, Primary Beneficiary [Member]
Fixed maturity securities, at fair value	3,315	3,354
Restricted Cash	2	2	$ 2
Financing Receivable, after Allowance for Credit Loss	2,943	2,998
Derivative assets	37	41
Other assets	2	2
Total assets	6,299	6,398
Long-term debt	4,270	4,324
Long-term Debt, at par less amortized discount	164	169
Long-term debt	4,434	4,493
Derivative Liability	1,800	1,835
Liabilities	6,234	6,328
Variable Interest Entity, Primary Beneficiary [Member] | Corporate Debt Securities [Member]
Fixed maturity securities, at fair value	3,175	3,215
Variable Interest Entity, Primary Beneficiary [Member] | Municipal Bonds [Member]
Debt Securities, Available-for-sale, Amortized Cost	110	113
Fixed maturity securities, at fair value	140	139
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	30	27
Variable Interest Entity, Primary Beneficiary [Member] | Ambac UK [Member]
Fixed maturity securities, at fair value	3,175	3,215
Restricted Cash	1	1
Financing Receivable, after Allowance for Credit Loss	2,943	2,998
Derivative assets	37	41
Other assets	0	0
Total assets	6,156	6,255
Long-term debt	4,270	4,324
Long-term Debt, at par less amortized discount	0	0
Long-term debt	4,270	4,324
Derivative Liability	1,800	1,835
Liabilities	6,070	6,159
Variable Interest Entity, Primary Beneficiary [Member] | Ambac UK [Member] | Corporate Debt Securities [Member]
Fixed maturity securities, at fair value	3,175	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	140	139
Restricted Cash	1	1
Financing Receivable, after Allowance for Credit Loss	0	0
Derivative assets	0	0
Other assets	2	2
Total assets	143	143
Long-term debt	0	0
Long-term Debt, at par less amortized discount	164	169
Long-term debt	164	169
Derivative Liability	0	0
Liabilities	164	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	$ 140	$ 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