Outstanding Exposure - Financial Guaranty Portfolio by Internal Rating (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Schedule of Insured Financial Obligations [Line Items]
Net	$ 236,383	$ 233,258
% of total net par outstanding	100.00%	100.00%
AAA
Schedule of Insured Financial Obligations [Line Items]
Net	$ 3,600	$ 3,584
% of total net par outstanding	1.50%	1.50%
AA
Schedule of Insured Financial Obligations [Line Items]
Net	$ 24,391	$ 24,383
% of total net par outstanding	10.30%	10.50%
A
Schedule of Insured Financial Obligations [Line Items]
Net	$ 110,321	$ 107,493
% of total net par outstanding	46.70%	46.10%
BBB
Schedule of Insured Financial Obligations [Line Items]
Net	$ 92,189	$ 91,906
% of total net par outstanding	39.00%	39.40%
BIG
Schedule of Insured Financial Obligations [Line Items]
Net	$ 5,882	$ 5,892
% of total net par outstanding	2.50%	2.50%
Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 226,746	$ 224,099
Public finance | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 180,837	$ 179,636
% of total net par outstanding	100.00%	100.00%
Public finance | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 45,909	$ 44,463
% of total net par outstanding	100.00%	100.00%
Public finance | AAA | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 220	$ 222
% of total net par outstanding	0.10%	0.10%
Public finance | AAA | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 2,000	$ 1,967
% of total net par outstanding	4.40%	4.40%
Public finance | AA | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 16,239	$ 16,241
% of total net par outstanding	9.00%	9.10%
Public finance | AA | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 3,535	$ 3,497
% of total net par outstanding	7.60%	7.90%
Public finance | A | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 97,909	$ 96,807
% of total net par outstanding	54.10%	53.90%
Public finance | A | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 10,460	$ 9,271
% of total net par outstanding	22.80%	20.90%
Public finance | BBB | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 62,665	$ 62,570
% of total net par outstanding	34.70%	34.80%
Public finance | BBB | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 28,924	$ 28,747
% of total net par outstanding	63.00%	64.60%
Public finance | BIG
Schedule of Insured Financial Obligations [Line Items]
Net	$ 4,794	$ 4,777
Public finance | BIG | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 3,804	$ 3,796
% of total net par outstanding	2.10%	2.10%
Public finance | BIG | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 990	$ 981
% of total net par outstanding	2.20%	2.20%
Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 9,637	$ 9,159
Structured finance | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 8,660	$ 8,228
% of total net par outstanding	100.00%	100.00%
Structured finance | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 977	$ 931
% of total net par outstanding	100.00%	100.00%
Structured finance | AAA | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 908	$ 926
% of total net par outstanding	10.50%	11.20%
Structured finance | AAA | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 472	$ 469
% of total net par outstanding	48.30%	50.40%
Structured finance | AA | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 4,605	$ 4,633
% of total net par outstanding	53.10%	56.30%
Structured finance | AA | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 12	$ 12
% of total net par outstanding	1.20%	1.30%
Structured finance | A | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 1,567	$ 1,075
% of total net par outstanding	18.10%	13.10%
Structured finance | A | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 385	$ 340
% of total net par outstanding	39.40%	36.50%
Structured finance | BBB | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 492	$ 479
% of total net par outstanding	5.70%	5.80%
Structured finance | BBB | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 108	$ 110
% of total net par outstanding	11.10%	11.80%
Structured finance | BIG
Schedule of Insured Financial Obligations [Line Items]
Net	$ 1,088	$ 1,115
Structured finance | BIG | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 1,088	$ 1,115
% of total net par outstanding	12.60%	13.60%
Structured finance | BIG | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 0	$ 0
% of total net par outstanding	0.00%	0.00%