Outstanding Exposure - Financial Guaranty Portfolio by Internal Rating (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 235,668	$ 234,153
% of total net par outstanding	100.00%	100.00%
AAA
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 3,883	$ 4,255
% of total net par outstanding	1.70%	1.80%
AA
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 25,478	$ 25,791
% of total net par outstanding	10.80%	11.00%
A
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 105,575	$ 103,703
% of total net par outstanding	44.80%	44.30%
BBB
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 93,386	$ 92,429
% of total net par outstanding	39.60%	39.50%
BIG
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 7,346	$ 7,975
% of total net par outstanding	3.10%	3.40%
Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 226,257	$ 224,625
Public finance | United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 175,952	$ 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	$ 50,305	$ 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	$ 308	$ 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	$ 2,342	$ 2,617
% of total net par outstanding	4.70%	4.90%
Public finance | AA | United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 16,613	$ 16,742
% of total net par outstanding	9.40%	9.70%
Public finance | AA | Non United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 4,208	$ 4,690
% of total net par outstanding	8.40%	8.80%
Public finance | A | United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 93,810	$ 90,914
% of total net par outstanding	53.30%	53.00%
Public finance | A | Non United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 10,638	$ 11,646
% of total net par outstanding	21.10%	22.00%
Public finance | BBB | United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 59,778	$ 58,162
% of total net par outstanding	34.00%	33.90%
Public finance | BBB | Non United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 32,640	$ 33,180
% of total net par outstanding	64.90%	62.60%
Public finance | BIG
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 5,920	$ 6,334
Public finance | BIG | United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 5,443	$ 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	$ 477	$ 895
% of total net par outstanding	0.90%	1.70%
Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 9,411	$ 9,528
Structured finance | United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 8,677	$ 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	$ 734	$ 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	$ 942	$ 1,146
% of total net par outstanding	10.90%	12.80%
Structured finance | AAA | Non United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 291	$ 152
% of total net par outstanding	39.60%	26.40%
Structured finance | AA | United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 4,647	$ 4,324
% of total net par outstanding	53.60%	48.30%
Structured finance | AA | Non United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 10	$ 35
% of total net par outstanding	1.40%	6.00%
Structured finance | A | United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 992	$ 1,006
% of total net par outstanding	11.40%	11.30%
Structured finance | A | Non United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 135	$ 137
% of total net par outstanding	18.40%	23.80%
Structured finance | BBB | United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 670	$ 835
% of total net par outstanding	7.70%	9.30%
Structured finance | BBB | Non United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 298	$ 252
% of total net par outstanding	40.60%	43.80%
Structured finance | BIG
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 1,426	$ 1,641
Structured finance | BIG | United States
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 1,426	$ 1,641
% of total net par outstanding	16.40%	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%