Revenues - Revenues disaggregated by Line of Business and Geographical Area (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue	$ 1,435	$ 1,214	$ 2,725	$ 2,356
United States
Disaggregation of Revenue [Line Items]
Revenue	837	638	1,551	1,250
Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	598	576	1,174	1,106
MIS
Disaggregation of Revenue [Line Items]
Revenue	938	739	1,732	1,409
MIS | United States
Disaggregation of Revenue [Line Items]
Revenue	616	436	1,119	847
MIS | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	322	303	613	562
MIS | Corporate Finance (CFG)
Disaggregation of Revenue [Line Items]
Revenue	572	388	1,025	743
MIS | Corporate Finance (CFG) | United States
Disaggregation of Revenue [Line Items]
Revenue	413	242	727	485
MIS | Corporate Finance (CFG) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	159	146	298	258
MIS | Structured Finance (SFG)
Disaggregation of Revenue [Line Items]
Revenue	81	112	177	213
MIS | Structured Finance (SFG) | United States
Disaggregation of Revenue [Line Items]
Revenue	45	72	106	134
MIS | Structured Finance (SFG) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	36	40	71	79
MIS | Financial Institutions (FIG)
Disaggregation of Revenue [Line Items]
Revenue	142	125	267	241
MIS | Financial Institutions (FIG) | United States
Disaggregation of Revenue [Line Items]
Revenue	70	52	130	98
MIS | Financial Institutions (FIG) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	72	73	137	143
MIS | Public, Project And Infrastructure Finance (PPIF)
Disaggregation of Revenue [Line Items]
Revenue	133	108	242	201
MIS | Public, Project And Infrastructure Finance (PPIF) | United States
Disaggregation of Revenue [Line Items]
Revenue	87	69	155	129
MIS | Public, Project And Infrastructure Finance (PPIF) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	46	39	87	72
MIS | Rating Revenue
Disaggregation of Revenue [Line Items]
Revenue	928	733	1,711	1,398
MIS | Rating Revenue | United States
Disaggregation of Revenue [Line Items]
Revenue	615	435	1,118	846
MIS | Rating Revenue | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	313	298	593	552
MIS | MIS Other
Disaggregation of Revenue [Line Items]
Revenue	10	6	21	11
MIS | MIS Other | United States
Disaggregation of Revenue [Line Items]
Revenue	1	1	1	1
MIS | MIS Other | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	9	5	20	10
MA
Disaggregation of Revenue [Line Items]
Revenue	497	475	993	947
MA | United States
Disaggregation of Revenue [Line Items]
Revenue	221	202	432	403
MA | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	276	273	561	544
MA | Research, Data And Analytics (RD&A)
Disaggregation of Revenue [Line Items]
Revenue	366	315	724	623
MA | Research, Data And Analytics (RD&A) | United States
Disaggregation of Revenue [Line Items]
Revenue	167	138	325	273
MA | Research, Data And Analytics (RD&A) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	199	177	399	350
MA | Enterprise Risk Solutions (ERS)
Disaggregation of Revenue [Line Items]
Revenue	131	117	269	239
MA | Enterprise Risk Solutions (ERS) | United States
Disaggregation of Revenue [Line Items]
Revenue	54	46	107	94
MA | Enterprise Risk Solutions (ERS) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	77	71	162	145
MA | Professional Services
Disaggregation of Revenue [Line Items]
Revenue	0	43	0	85
MA | Professional Services | United States
Disaggregation of Revenue [Line Items]
Revenue	0	18	0	36
MA | Professional Services | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	$ 0	$ 25	$ 0	$ 49