Revenues - Revenues disaggregated by Line of Business and Geographical Area (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue	$ 1,356	$ 1,240	$ 4,081	$ 3,596
United States
Disaggregation of Revenue [Line Items]
Revenue	729	660	2,280	1,910
Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	627	580	1,801	1,686
MIS
Disaggregation of Revenue [Line Items]
Revenue	825	746	2,557	2,155
MIS | United States
Disaggregation of Revenue [Line Items]
Revenue	506	454	1,625	1,301
MIS | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	319	292	932	854
MIS | Corporate Finance (CFG)
Disaggregation of Revenue [Line Items]
Revenue	461	392	1,486	1,135
MIS | Corporate Finance (CFG) | United States
Disaggregation of Revenue [Line Items]
Revenue	311	259	1,038	744
MIS | Corporate Finance (CFG) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	150	133	448	391
MIS | Structured Finance (SFG)
Disaggregation of Revenue [Line Items]
Revenue	88	105	265	318
MIS | Structured Finance (SFG) | United States
Disaggregation of Revenue [Line Items]
Revenue	54	68	160	202
MIS | Structured Finance (SFG) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	34	37	105	116
MIS | Financial Institutions (FIG)
Disaggregation of Revenue [Line Items]
Revenue	134	120	401	361
MIS | Financial Institutions (FIG) | United States
Disaggregation of Revenue [Line Items]
Revenue	59	54	189	152
MIS | Financial Institutions (FIG) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	75	66	212	209
MIS | Public, Project And Infrastructure Finance (PPIF)
Disaggregation of Revenue [Line Items]
Revenue	133	120	375	321
MIS | Public, Project And Infrastructure Finance (PPIF) | United States
Disaggregation of Revenue [Line Items]
Revenue	82	73	237	202
MIS | Public, Project And Infrastructure Finance (PPIF) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	51	47	138	119
MIS | Rating Revenue
Disaggregation of Revenue [Line Items]
Revenue	816	737	2,527	2,135
MIS | Rating Revenue | United States
Disaggregation of Revenue [Line Items]
Revenue	506	454	1,624	1,300
MIS | Rating Revenue | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	310	283	903	835
MIS | MIS Other
Disaggregation of Revenue [Line Items]
Revenue	9	9	30	20
MIS | MIS Other | United States
Disaggregation of Revenue [Line Items]
Revenue	0	0	1	1
MIS | MIS Other | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	9	9	29	19
MA
Disaggregation of Revenue [Line Items]
Revenue	531	494	1,524	1,441
MA | United States
Disaggregation of Revenue [Line Items]
Revenue	223	206	655	609
MA | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	308	288	869	832
MA | Research, Data And Analytics (RD&A)
Disaggregation of Revenue [Line Items]
Revenue	386	317	1,110	940
MA | Research, Data And Analytics (RD&A) | United States
Disaggregation of Revenue [Line Items]
Revenue	167	139	492	412
MA | Research, Data And Analytics (RD&A) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	219	178	618	528
MA | Enterprise Risk Solutions (ERS)
Disaggregation of Revenue [Line Items]
Revenue	145	134	414	373
MA | Enterprise Risk Solutions (ERS) | United States
Disaggregation of Revenue [Line Items]
Revenue	56	49	163	143
MA | Enterprise Risk Solutions (ERS) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	89	85	251	230
MA | Professional Services
Disaggregation of Revenue [Line Items]
Revenue	0	43	0	128
MA | Professional Services | United States
Disaggregation of Revenue [Line Items]
Revenue	0	18	0	54
MA | Professional Services | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	$ 0	$ 25	$ 0	$ 74