Revenues - Revenues Disaggregated by Line of Business and Geographical Area (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
United States
Disaggregation of Revenue [Line Items]
Revenue	831	837	1,716	1,551
Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	722	598	1,437	1,174
MIS
Disaggregation of Revenue [Line Items]
Revenue	980	938	2,016	1,732
MIS | United States
Disaggregation of Revenue [Line Items]
Revenue	582	616	1,229	1,119
MIS | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	398	322	787	613
MIS | Corporate Finance (CFG)
Disaggregation of Revenue [Line Items]
Revenue	550	572	1,155	1,025
MIS | Corporate Finance (CFG) | United States
Disaggregation of Revenue [Line Items]
Revenue	345	413	759	727
MIS | Corporate Finance (CFG) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	205	159	396	298
MIS | Financial Institutions (FIG)
Disaggregation of Revenue [Line Items]
Revenue	150	142	312	267
MIS | Financial Institutions (FIG) | United States
Disaggregation of Revenue [Line Items]
Revenue	69	70	155	130
MIS | Financial Institutions (FIG) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	81	72	157	137
MIS | Public, Project And Infrastructure Finance (PPIF)
Disaggregation of Revenue [Line Items]
Revenue	130	133	273	242
MIS | Public, Project And Infrastructure Finance (PPIF) | United States
Disaggregation of Revenue [Line Items]
Revenue	79	87	157	155
MIS | Public, Project And Infrastructure Finance (PPIF) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	51	46	116	87
MIS | Structured Finance (SFG)
Disaggregation of Revenue [Line Items]
Revenue	140	81	256	177
MIS | Structured Finance (SFG) | United States
Disaggregation of Revenue [Line Items]
Revenue	88	45	156	106
MIS | Structured Finance (SFG) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	52	36	100	71
MIS | Rating Revenue
Disaggregation of Revenue [Line Items]
Revenue	970	928	1,996	1,711
MIS | Rating Revenue | United States
Disaggregation of Revenue [Line Items]
Revenue	581	615	1,227	1,118
MIS | Rating Revenue | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	389	313	769	593
MIS | MIS Other
Disaggregation of Revenue [Line Items]
Revenue	10	10	20	21
MIS | MIS Other | United States
Disaggregation of Revenue [Line Items]
Revenue	1	1	2	1
MIS | MIS Other | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	9	9	18	20
MA
Disaggregation of Revenue [Line Items]
Revenue	573	497	1,137	993
MA | United States
Disaggregation of Revenue [Line Items]
Revenue	249	221	487	432
MA | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	324	276	650	561
MA | Research, Data And Analytics (RD&A)
Disaggregation of Revenue [Line Items]
Revenue	435	366	854	724
MA | Research, Data And Analytics (RD&A) | United States
Disaggregation of Revenue [Line Items]
Revenue	190	167	373	325
MA | Research, Data And Analytics (RD&A) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	245	199	481	399
MA | Enterprise Risk Solutions (ERS)
Disaggregation of Revenue [Line Items]
Revenue	138	131	283	269
MA | Enterprise Risk Solutions (ERS) | United States
Disaggregation of Revenue [Line Items]
Revenue	59	54	114	107
MA | Enterprise Risk Solutions (ERS) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	$ 79	$ 77	$ 169	$ 162