Variable Interest Entities Special Purpose Entities, including Variable Interest Entities - Summary of Assets and Liabilities (Details) $ in Millions	Jun. 30, 2020 USD ($) Entity	Dec. 31, 2019 USD ($) Entity	Jun. 30, 2019 USD ($)
Debt Securities, Available-for-sale, Amortized Cost	$ 2,972	$ 3,187
Fixed income securities, at fair value (amortized cost of $2,126 and $2,450)	3,065
Restricted Cash	15	55	$ 0
Derivative assets	97	75
Other assets	145	95
Total assets	12,761	13,320
Interest Payable	474	441
Long-term debt	2,749	2,822
Derivative Liability	134	90
Other Liabilities	83	93
Liabilities	11,632	11,783
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	113	132
Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,126 and $2,450)	3,041	3,121
Restricted Cash	2	2	$ 29
Financing Receivable, after Allowance for Credit Loss	2,787	3,108
Derivative assets	61	52
Other assets	2	3
Total assets	5,893	6,286
Interest Payable	0	1
Long-term debt	3,953	4,351
Long-term Debt, at par less amortized discount	173	203
Long-term debt	4,125	4,554
Derivative Liability	1,709	1,657
Liabilities	5,835	6,212
Variable Interest Entity, Primary Beneficiary [Member] | Corporate Debt Securities [Member]
Fixed income securities, at fair value (amortized cost of $2,126 and $2,450)	2,907	2,957
Variable Interest Entity, Primary Beneficiary [Member] | Municipal Bonds [Member]
Debt Securities, Available-for-sale, Amortized Cost	116	139
Fixed income securities, at fair value (amortized cost of $2,126 and $2,450)	134	164
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	18	25
Variable Interest Entity, Primary Beneficiary [Member] | Ambac UK [Member]
Fixed income securities, at fair value (amortized cost of $2,126 and $2,450)	2,907	2,957
Restricted Cash	1	1
Financing Receivable, after Allowance for Credit Loss	2,787	3,108
Derivative assets	61	52
Other assets	0	1
Total assets	5,756	6,119
Interest Payable	0	1
Long-term debt	3,953	4,351
Long-term Debt, at par less amortized discount	0	0
Long-term debt	3,953	4,351
Derivative Liability	1,709	1,657
Liabilities	5,662	6,009
Variable Interest Entity, Primary Beneficiary [Member] | Ambac UK [Member] | Corporate Debt Securities [Member]
Fixed income securities, at fair value (amortized cost of $2,126 and $2,450)	2,907	2,957
Variable Interest Entity, Primary Beneficiary [Member] | Ambac UK [Member] | Municipal Bonds [Member]
Fixed income securities, at fair value (amortized cost of $2,126 and $2,450)	0	0
Variable Interest Entity, Primary Beneficiary [Member] | Ambac Assurance [Member]
Fixed income securities, at fair value (amortized cost of $2,126 and $2,450)	134	164
Restricted Cash	1	1
Financing Receivable, after Allowance for Credit Loss	0	0
Derivative assets	0	0
Other assets	2	2
Total assets	138	167
Interest Payable	0	0
Long-term debt	0	0
Long-term Debt, at par less amortized discount	173	203
Long-term debt	173	203
Derivative Liability	0	0
Liabilities	173	203
Variable Interest Entity, Primary Beneficiary [Member] | Ambac Assurance [Member] | Corporate Debt Securities [Member]
Fixed income securities, at fair value (amortized cost of $2,126 and $2,450)	0	0
Variable Interest Entity, Primary Beneficiary [Member] | Ambac Assurance [Member] | Municipal Bonds [Member]
Fixed income securities, at fair value (amortized cost of $2,126 and $2,450)	$ 134	$ 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