Outstanding Exposure - Financial Guaranty Portfolio by Internal Rating (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 234,270	$ 234,153
% of total net par outstanding	100.00%	100.00%
AAA
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 4,258	$ 4,255
% of total net par outstanding	1.80%	1.80%
AA
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 25,317	$ 25,791
% of total net par outstanding	10.80%	11.00%
A
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 104,568	$ 103,703
% of total net par outstanding	44.60%	44.30%
BBB
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 92,618	$ 92,429
% of total net par outstanding	39.60%	39.50%
BIG
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 7,509	$ 7,975
% of total net par outstanding	3.20%	3.40%
Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 225,040	$ 224,625
Public finance | United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 172,941	$ 171,597
% of total net par outstanding	100.00%	100.00%
Public finance | Non United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 52,099	$ 53,028
% of total net par outstanding	100.00%	100.00%
Public finance | AAA | United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 338	$ 340
% of total net par outstanding	0.20%	0.20%
Public finance | AAA | Non United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 2,626	$ 2,617
% of total net par outstanding	5.00%	4.90%
Public finance | AA | United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 16,451	$ 16,742
% of total net par outstanding	9.60%	9.70%
Public finance | AA | Non United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 4,629	$ 4,690
% of total net par outstanding	8.90%	8.80%
Public finance | A | United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 92,060	$ 90,914
% of total net par outstanding	53.20%	53.00%
Public finance | A | Non United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 11,439	$ 11,646
% of total net par outstanding	22.00%	22.00%
Public finance | BBB | United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 58,694	$ 58,162
% of total net par outstanding	33.90%	33.90%
Public finance | BBB | Non United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 32,890	$ 33,180
% of total net par outstanding	63.10%	62.60%
Public finance | BIG
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 5,913	$ 6,334
Public finance | BIG | United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 5,398	$ 5,439
% of total net par outstanding	3.10%	3.20%
Public finance | BIG | Non United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 515	$ 895
% of total net par outstanding	1.00%	1.70%
Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 9,230	$ 9,528
Structured finance | United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 8,678	$ 8,952
% of total net par outstanding	100.00%	100.00%
Structured finance | Non United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 552	$ 576
% of total net par outstanding	100.00%	100.00%
Structured finance | AAA | United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 1,142	$ 1,146
% of total net par outstanding	13.20%	12.80%
Structured finance | AAA | Non United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 152	$ 152
% of total net par outstanding	27.50%	26.40%
Structured finance | AA | United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 4,203	$ 4,324
% of total net par outstanding	48.40%	48.30%
Structured finance | AA | Non United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 34	$ 35
% of total net par outstanding	6.20%	6.00%
Structured finance | A | United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 931	$ 1,006
% of total net par outstanding	10.70%	11.30%
Structured finance | A | Non United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 138	$ 137
% of total net par outstanding	25.00%	23.80%
Structured finance | BBB | United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 806	$ 835
% of total net par outstanding	9.30%	9.30%
Structured finance | BBB | Non United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 228	$ 252
% of total net par outstanding	41.30%	43.80%
Structured finance | BIG
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 1,596	$ 1,641
Structured finance | BIG | United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 1,596	$ 1,641
% of total net par outstanding	18.40%	18.30%
Structured finance | BIG | Non United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 0	$ 0
% of total net par outstanding	0.00%	0.00%