Variable Interest Entities Special Purpose Entities, including Variable Interest Entities - Summary of Assets and Liabilities (Details) $ in Millions	Dec. 31, 2021 USD ($) Entity	Dec. 31, 2020 USD ($) Entity	Dec. 31, 2019 USD ($)
Available-for-sale Securities, Amortized Cost Basis	$ 2,140	$ 2,807
Available-for-sale Securities	2,265
Restricted Cash	5	13	$ 55
Derivative assets	76	93
Other assets	68	68
Assets	12,303	13,220
Accrued interest payable	576	517
Long-term debt	2,230	2,739
Derivative liabilities	95	114
Liabilities	11,187	12,074
Variable Interest Entity [Member]
Available-for-sale Securities	3,455	3,354
Restricted Cash	2	2	$ 2
Loans, at fair value	2,718	2,998
Derivative assets	38	41
Other assets	2	2
Assets	6,216	6,398
Long-term Debt	4,056	4,324
Long-term Debt, at par less amortized discount	160	169
Long-term debt	4,216	4,493
Derivative liabilities	1,940	1,835
Liabilities	6,156	6,328
Loans, Unpaid principal balance	2,363	2,546
Long-term debt, Unpaid principal balance	3,579	3,769
Variable Interest Entity [Member] | Ambac UK [Member]
Available-for-sale Securities	3,320	3,215
Restricted Cash	1	1
Loans, at fair value	2,718	2,998
Derivative assets	38	41
Other assets	0	0
Assets	6,077	6,255
Long-term Debt	4,056	4,324
Long-term Debt, at par less amortized discount	0	0
Long-term debt	4,056	4,324
Derivative liabilities	1,940	1,835
Liabilities	5,996	6,159
Variable Interest Entity [Member] | Ambac Assurance [Member]
Available-for-sale Securities	136	139
Restricted Cash	1	1
Loans, at fair value	0	0
Derivative assets	0	0
Other assets	2	2
Assets	139	143
Long-term Debt	0	0
Long-term Debt, at par less amortized discount	160	169
Long-term debt	160	169
Derivative liabilities	0	0
Liabilities	160	169
Variable Interest Entity [Member] | Corporate Debt Securities [Member]
Available-for-sale Securities	3,320	3,215
Variable Interest Entity [Member] | Corporate Debt Securities [Member] | Ambac UK [Member]
Available-for-sale Securities	3,320	3,215
Variable Interest Entity [Member] | Corporate Debt Securities [Member] | Ambac Assurance [Member]
Available-for-sale Securities	0	0
Variable Interest Entity [Member] | Municipal Bonds [Member]
Available-for-sale Securities, Amortized Cost Basis	106	113
Available-for-sale Securities	136	139
Variable Interest Entity [Member] | Municipal Bonds [Member] | Ambac UK [Member]
Available-for-sale Securities	0	0
Variable Interest Entity [Member] | Municipal Bonds [Member] | Ambac Assurance [Member]
Available-for-sale Securities	$ 136	$ 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