Variable Interest Entities Special Purpose Entities, including Variable Interest Entities - Summary of Assets and Liabilities (Details) $ in Millions	Sep. 30, 2020 USD ($) Entity	Dec. 31, 2019 USD ($) Entity	Sep. 30, 2019 USD ($)
Debt Securities, Available-for-sale, Amortized Cost	$ 2,914
Fixed income securities, at fair value	3,048
Restricted Cash	10	$ 55	$ 13
Derivative assets	95	75
Other assets	60	95
Total assets	12,812	13,320
Interest Payable	499	441
Long-term debt	2,737	2,822
Derivative Liability	126	90
Other Liabilities	91	93
Liabilities	11,718	11,783
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	141	132
Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value	3,160	3,121
Restricted Cash	2	2	$ 3
Financing Receivable, after Allowance for Credit Loss	2,783	3,108
Derivative assets	53	52
Other assets	1	3
Total assets	5,998	6,286
Interest Payable	0	1
Long-term debt	4,002	4,351
Long-term Debt, at par less amortized discount	167	203
Long-term debt	4,169	4,554
Derivative Liability	1,771	1,657
Liabilities	5,940	6,212
Variable Interest Entity, Primary Beneficiary [Member] | Corporate Debt Securities [Member]
Fixed income securities, at fair value	3,029	2,957
Variable Interest Entity, Primary Beneficiary [Member] | Municipal Bonds [Member]
Debt Securities, Available-for-sale, Amortized Cost	112	139
Fixed income securities, at fair value	131	164
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	19	25
Variable Interest Entity, Primary Beneficiary [Member] | Ambac UK [Member]
Fixed income securities, at fair value	3,029	2,957
Restricted Cash	1	1
Financing Receivable, after Allowance for Credit Loss	2,783	3,108
Derivative assets	53	52
Other assets	0	1
Total assets	5,865	6,119
Interest Payable	0	1
Long-term debt	4,002	4,351
Long-term Debt, at par less amortized discount	0	0
Long-term debt	4,002	4,351
Derivative Liability	1,771	1,657
Liabilities	5,773	6,009
Variable Interest Entity, Primary Beneficiary [Member] | Ambac UK [Member] | Corporate Debt Securities [Member]
Fixed income securities, at fair value	3,029	2,957
Variable Interest Entity, Primary Beneficiary [Member] | Ambac UK [Member] | Municipal Bonds [Member]
Fixed income securities, at fair value	0	0
Variable Interest Entity, Primary Beneficiary [Member] | Ambac Assurance [Member]
Fixed income securities, at fair value	131	164
Restricted Cash	1	1
Financing Receivable, after Allowance for Credit Loss	0	0
Derivative assets	0	0
Other assets	1	2
Total assets	133	167
Interest Payable	0	0
Long-term debt	0	0
Long-term Debt, at par less amortized discount	167	203
Long-term debt	167	203
Derivative Liability	0	0
Liabilities	167	203
Variable Interest Entity, Primary Beneficiary [Member] | Ambac Assurance [Member] | Corporate Debt Securities [Member]
Fixed income securities, at fair value	0	0
Variable Interest Entity, Primary Beneficiary [Member] | Ambac Assurance [Member] | Municipal Bonds [Member]
Fixed income securities, at fair value	$ 131	$ 164
Consolidated Entities [Member]
Number Of Consolidated Variable Interest Entities | Entity	6	7
Consolidated Entities [Member] | Ambac UK [Member]
Number Of Consolidated Variable Interest Entities | Entity	5	6
Consolidated Entities [Member] | Ambac Assurance [Member]
Number Of Consolidated Variable Interest Entities | Entity	1	1