Variable Interest Entities Special Purpose Entities, including Variable Interest Entities - Summary of Assets and Liabilities (Details) $ in Millions	Dec. 31, 2020 USD ($) Entity	Dec. 31, 2019 USD ($) Entity	Dec. 31, 2018 USD ($)
Available-for-sale Securities, Amortized Cost Basis	$ 2,807
Available-for-sale Securities	2,949	$ 3,314
Restricted Cash	13	55	$ 19
Derivative assets	93	75
Other assets	114	95
Assets	13,220	13,320
Accrued interest payable	517	441
Long-term debt	2,739	2,822
Derivative liabilities	114	90
Liabilities	12,074	11,783
Ambac Assurance [Member]
Restricted Cash	9	55
Variable Interest Entity [Member]
Available-for-sale Securities	3,354	3,121
Restricted Cash	2	2	$ 1
Loans, at fair value	2,998	3,108
Derivative assets	41	52
Other assets	2	3
Assets	6,398	6,286
Accrued interest payable	0	1
Long-term Debt	4,324	4,351
Long-term Debt, at par less amortized discount	169	203
Long-term debt	4,493	4,554
Derivative liabilities	1,835	1,657
Liabilities	6,328	6,212
Variable Interest Entity [Member] | Ambac UK [Member]
Available-for-sale Securities	3,215	2,957
Restricted Cash	1	1
Loans, at fair value	2,998	3,108
Derivative assets	41	52
Other assets	0	1
Assets	6,255	6,119
Accrued interest payable	0	1
Long-term Debt	4,324	4,351
Long-term Debt, at par less amortized discount	0	0
Long-term debt	4,324	4,351
Derivative liabilities	1,835	1,657
Liabilities	6,159	6,009
Variable Interest Entity [Member] | Ambac Assurance [Member]
Available-for-sale Securities	139	164
Restricted Cash	1	1
Loans, at fair value	0	0
Derivative assets	0	0
Other assets	2	2
Assets	143	167
Accrued interest payable	0	0
Long-term Debt	0	0
Long-term Debt, at par less amortized discount	169	203
Long-term debt	169	203
Derivative liabilities	0	0
Liabilities	169	203
Variable Interest Entity [Member] | Corporate Debt Securities [Member]
Available-for-sale Securities	3,215	2,957
Variable Interest Entity [Member] | Corporate Debt Securities [Member] | Ambac UK [Member]
Available-for-sale Securities	3,215	2,957
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	113	139
Available-for-sale Securities	139	164
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	$ 139	$ 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