Revenues - Revenues Disaggregated by Line of Business and Geographical Area (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Revenue	$ 1,526	$ 1,356	$ 4,679	$ 4,081
United States
Disaggregation of Revenue [Line Items]
Revenue	844	729	2,560	2,280
Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	682	627	2,119	1,801
MIS
Disaggregation of Revenue [Line Items]
Revenue	925	825	2,941	2,557
MIS | United States
Disaggregation of Revenue [Line Items]
Revenue	580	506	1,809	1,625
MIS | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	345	319	1,132	932
MIS | Corporate Finance (CFG)
Disaggregation of Revenue [Line Items]
Revenue	488	461	1,643	1,486
MIS | Corporate Finance (CFG) | United States
Disaggregation of Revenue [Line Items]
Revenue	334	311	1,093	1,038
MIS | Corporate Finance (CFG) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	154	150	550	448
MIS | Financial Institutions (FIG)
Disaggregation of Revenue [Line Items]
Revenue	153	134	465	401
MIS | Financial Institutions (FIG) | United States
Disaggregation of Revenue [Line Items]
Revenue	71	59	226	189
MIS | Financial Institutions (FIG) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	82	75	239	212
MIS | Public, Project And Infrastructure Finance (PPIF)
Disaggregation of Revenue [Line Items]
Revenue	130	133	403	375
MIS | Public, Project And Infrastructure Finance (PPIF) | United States
Disaggregation of Revenue [Line Items]
Revenue	76	82	233	237
MIS | Public, Project And Infrastructure Finance (PPIF) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	54	51	170	138
MIS | Structured Finance (SFG)
Disaggregation of Revenue [Line Items]
Revenue	143	88	399	265
MIS | Structured Finance (SFG) | United States
Disaggregation of Revenue [Line Items]
Revenue	98	54	254	160
MIS | Structured Finance (SFG) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	45	34	145	105
MIS | Rating Revenue
Disaggregation of Revenue [Line Items]
Revenue	914	816	2,910	2,527
MIS | Rating Revenue | United States
Disaggregation of Revenue [Line Items]
Revenue	579	506	1,806	1,624
MIS | Rating Revenue | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	335	310	1,104	903
MIS | MIS Other
Disaggregation of Revenue [Line Items]
Revenue	11	9	31	30
MIS | MIS Other | United States
Disaggregation of Revenue [Line Items]
Revenue	1	0	3	1
MIS | MIS Other | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	10	9	28	29
MA
Disaggregation of Revenue [Line Items]
Revenue	601	531	1,738	1,524
MA | United States
Disaggregation of Revenue [Line Items]
Revenue	264	223	751	655
MA | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	337	308	987	869
MA | Research, Data And Analytics (RD&A)
Disaggregation of Revenue [Line Items]
Revenue	445	386	1,299	1,110
MA | Research, Data And Analytics (RD&A) | United States
Disaggregation of Revenue [Line Items]
Revenue	203	167	576	492
MA | Research, Data And Analytics (RD&A) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	242	219	723	618
MA | Enterprise Risk Solutions (ERS)
Disaggregation of Revenue [Line Items]
Revenue	156	145	439	414
MA | Enterprise Risk Solutions (ERS) | United States
Disaggregation of Revenue [Line Items]
Revenue	61	56	175	163
MA | Enterprise Risk Solutions (ERS) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	$ 95	$ 89	$ 264	$ 251