Revenues - Revenue by Category (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Revenue	$ 1,553	$ 1,435	$ 3,153	$ 2,725
Eliminations
Disaggregation of Revenue [Line Items]
Revenue	(44)	(37)	(86)	(76)
MIS
Disaggregation of Revenue [Line Items]
Revenue	980	938	2,016	1,732
MIS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	1,022	973	2,098	1,804
MIS | Eliminations
Disaggregation of Revenue [Line Items]
Revenue	42	35	82	72
MIS | Corporate Finance (CFG)
Disaggregation of Revenue [Line Items]
Revenue	550	572	1,155	1,025
MIS | Corporate Finance (CFG) | Investment-grade
Disaggregation of Revenue [Line Items]
Revenue	102	291	236	435
MIS | Corporate Finance (CFG) | High-yield
Disaggregation of Revenue [Line Items]
Revenue	124	99	265	174
MIS | Corporate Finance (CFG) | Bank loans
Disaggregation of Revenue [Line Items]
Revenue	157	43	337	132
MIS | Corporate Finance (CFG) | Other accounts
Disaggregation of Revenue [Line Items]
Revenue	167	139	317	284
MIS | Financial Institutions (FIG)
Disaggregation of Revenue [Line Items]
Revenue	150	142	312	267
MIS | Financial Institutions (FIG) | Other accounts
Disaggregation of Revenue [Line Items]
Revenue	3	2	5	5
MIS | Financial Institutions (FIG) | Banking
Disaggregation of Revenue [Line Items]
Revenue	101	88	210	174
MIS | Financial Institutions (FIG) | Insurance
Disaggregation of Revenue [Line Items]
Revenue	33	44	76	74
MIS | Financial Institutions (FIG) | Managed investments
Disaggregation of Revenue [Line Items]
Revenue	13	8	21	14
MIS | Public, Project And Infrastructure Finance (PPIF)
Disaggregation of Revenue [Line Items]
Revenue	130	133	273	242
MIS | Public, Project And Infrastructure Finance (PPIF) | Public finance / sovereign
Disaggregation of Revenue [Line Items]
Revenue	63	64	130	121
MIS | Public, Project And Infrastructure Finance (PPIF) | Project and infrastructure
Disaggregation of Revenue [Line Items]
Revenue	67	69	143	121
MIS | Structured Finance (SFG)
Disaggregation of Revenue [Line Items]
Revenue	140	81	256	177
MIS | Structured Finance (SFG) | Other accounts
Disaggregation of Revenue [Line Items]
Revenue	0	1	1	2
MIS | Structured Finance (SFG) | Asset-backed securities
Disaggregation of Revenue [Line Items]
Revenue	33	23	59	45
MIS | Structured Finance (SFG) | RMBS
Disaggregation of Revenue [Line Items]
Revenue	31	23	58	50
MIS | Structured Finance (SFG) | CMBS
Disaggregation of Revenue [Line Items]
Revenue	23	13	47	30
MIS | Structured Finance (SFG) | Structured credit
Disaggregation of Revenue [Line Items]
Revenue	53	21	91	50
MIS | Rating Revenue
Disaggregation of Revenue [Line Items]
Revenue	970	928	1,996	1,711
MIS | MIS Other
Disaggregation of Revenue [Line Items]
Revenue	10	10	20	21
MA
Disaggregation of Revenue [Line Items]
Revenue	573	497	1,137	993
MA | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	575	499	1,141	997
MA | Eliminations
Disaggregation of Revenue [Line Items]
Revenue	2	2	4	4
MA | Research, Data And Analytics (RD&A)
Disaggregation of Revenue [Line Items]
Revenue	435	366	854	724
MA | Enterprise Risk Solutions (ERS)
Disaggregation of Revenue [Line Items]
Revenue	$ 138	$ 131	$ 283	$ 269