Variable Interest Entities Special Purpose Entities, including Variable Interest Entities - Summary of Assets and Liabilities (Details) $ in Millions	Mar. 31, 2020 USD ($) Entity	Dec. 31, 2019 USD ($) Entity	Mar. 31, 2019 USD ($)
Debt Securities, Available-for-sale, Amortized Cost	$ 3,038	$ 3,187
Fixed income securities, at fair value (amortized cost of $2,367 and $2,450)	3,037
Restricted Cash	31	55	$ 0
Derivative assets	88	75
Other assets	144	95
Total assets	12,777	13,320
Interest Payable	457	441
Long-term debt	2,760	2,822
Derivative Liability	137	90
Other Liabilities	131	93
Liabilities	11,716	11,783
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	81	132
Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,367 and $2,450)	2,928	3,121
Restricted Cash	2	2	$ 3
Financing Receivable, after Allowance for Credit Loss	2,932	3,108
Derivative assets	62	52
Other assets	1	3
Total assets	5,925	6,286
Interest Payable	0	1
Long-term debt	4,092	4,351
Long-term Debt, at par less amortized discount	171	203
Long-term debt	4,263	4,554
Derivative Liability	1,610	1,657
Liabilities	5,873	6,212
Corporate Debt Securities [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,367 and $2,450)	2,806	2,957
Municipal Bonds [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Debt Securities, Available-for-sale, Amortized Cost	116	139
Fixed income securities, at fair value (amortized cost of $2,367 and $2,450)	122	164
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	6	25
Ambac UK [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,367 and $2,450)	2,806	2,957
Restricted Cash	1	1
Financing Receivable, after Allowance for Credit Loss	2,932	3,108
Derivative assets	62	52
Other assets	0	1
Total assets	5,801	6,119
Interest Payable	0	1
Long-term debt	4,092	4,351
Long-term Debt, at par less amortized discount	0	0
Long-term debt	4,092	4,351
Derivative Liability	1,610	1,657
Liabilities	5,702	6,009
Ambac UK [Member] | Corporate Debt Securities [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,367 and $2,450)	2,806	2,957
Ambac UK [Member] | Municipal Bonds [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,367 and $2,450)	0	0
Ambac Assurance [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,367 and $2,450)	122	164
Restricted Cash	1	1
Financing Receivable, after Allowance for Credit Loss	0	0
Derivative assets	0	0
Other assets	1	2
Total assets	124	167
Interest Payable	0	0
Long-term debt	0	0
Long-term Debt, at par less amortized discount	171	203
Long-term debt	171	203
Derivative Liability	0	0
Liabilities	171	203
Ambac Assurance [Member] | Corporate Debt Securities [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,367 and $2,450)	0	0
Ambac Assurance [Member] | Municipal Bonds [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,367 and $2,450)	$ 122	$ 164
Consolidated Entities [Member]
Number Of Consolidated Variable Interest Entities | Entity	7	7
Consolidated Entities [Member] | Ambac UK [Member]
Number Of Consolidated Variable Interest Entities | Entity	6	6
Consolidated Entities [Member] | Ambac Assurance [Member]
Number Of Consolidated Variable Interest Entities | Entity	1	1