Revenues - Revenues disaggregated by Line of Business and Geographical Area (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenue	$ 1,290	$ 1,142
United States
Disaggregation of Revenue [Line Items]
Revenue	714	612
Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	576	530
MIS
Disaggregation of Revenue [Line Items]
Revenue	794	670
MIS | United States
Disaggregation of Revenue [Line Items]
Revenue	503	411
MIS | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	291	259
MIS | Corporate Finance (CFG)
Disaggregation of Revenue [Line Items]
Revenue	453	355
MIS | Corporate Finance (CFG) | United States
Disaggregation of Revenue [Line Items]
Revenue	314	243
MIS | Corporate Finance (CFG) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	139	112
MIS | Structured Finance (SFG)
Disaggregation of Revenue [Line Items]
Revenue	96	101
MIS | Structured Finance (SFG) | United States
Disaggregation of Revenue [Line Items]
Revenue	61	62
MIS | Structured Finance (SFG) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	35	39
MIS | Financial Institutions (FIG)
Disaggregation of Revenue [Line Items]
Revenue	125	116
MIS | Financial Institutions (FIG) | United States
Disaggregation of Revenue [Line Items]
Revenue	60	46
MIS | Financial Institutions (FIG) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	65	70
MIS | Public, Project And Infrastructure Finance (PPIF)
Disaggregation of Revenue [Line Items]
Revenue	109	93
MIS | Public, Project And Infrastructure Finance (PPIF) | United States
Disaggregation of Revenue [Line Items]
Revenue	68	60
MIS | Public, Project And Infrastructure Finance (PPIF) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	41	33
MIS | Rating Revenue
Disaggregation of Revenue [Line Items]
Revenue	783	665
MIS | Rating Revenue | United States
Disaggregation of Revenue [Line Items]
Revenue	503	411
MIS | Rating Revenue | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	280	254
MIS | MIS Other
Disaggregation of Revenue [Line Items]
Revenue	11	5
MIS | MIS Other | United States
Disaggregation of Revenue [Line Items]
Revenue	0	0
MIS | MIS Other | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	11	5
MA
Disaggregation of Revenue [Line Items]
Revenue	496	472
MA | United States
Disaggregation of Revenue [Line Items]
Revenue	211	201
MA | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	285	271
MA | Research, Data And Analytics (RD&A)
Disaggregation of Revenue [Line Items]
Revenue	358	308
MA | Research, Data And Analytics (RD&A) | United States
Disaggregation of Revenue [Line Items]
Revenue	158	135
MA | Research, Data And Analytics (RD&A) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	200	173
MA | Enterprise Risk Solutions (ERS)
Disaggregation of Revenue [Line Items]
Revenue	138	122
MA | Enterprise Risk Solutions (ERS) | United States
Disaggregation of Revenue [Line Items]
Revenue	53	48
MA | Enterprise Risk Solutions (ERS) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	85	74
MA | Professional Services
Disaggregation of Revenue [Line Items]
Revenue	0	42
MA | Professional Services | United States
Disaggregation of Revenue [Line Items]
Revenue	0	18
MA | Professional Services | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	$ 0	$ 24