Revenues - Revenues disaggregated by Line of Business and Geographical Area (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Revenue	$ 1,233	$ 1,240	$ 1,214	$ 1,142	$ 1,060	$ 1,081	$ 1,175	$ 1,127	$ 4,829	$ 4,443	$ 4,204
U.S.
Disaggregation of Revenue [Line Items]
Revenue									2,544	2,330	2,348
Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue									2,285	2,113	1,856
MIS
Disaggregation of Revenue [Line Items]
Revenue									2,875	2,712	2,774
MIS | U.S.
Disaggregation of Revenue [Line Items]
Revenue									1,721	1,619	1,702
MIS | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue									1,154	1,093	1,072
MIS | Corporate finance (CFG)
Disaggregation of Revenue [Line Items]
Revenue									1,497	1,379	1,448
MIS | Corporate finance (CFG) | U.S.
Disaggregation of Revenue [Line Items]
Revenue									968	894	961
MIS | Corporate finance (CFG) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue									529	485	487
MIS | Structured finance (SFG)
Disaggregation of Revenue [Line Items]
Revenue									427	481	440
MIS | Structured finance (SFG) | U.S.
Disaggregation of Revenue [Line Items]
Revenue									270	301	288
MIS | Structured finance (SFG) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue									157	180	152
MIS | Financial institutions (FIG)
Disaggregation of Revenue [Line Items]
Revenue									476	442	436
MIS | Financial institutions (FIG) | U.S.
Disaggregation of Revenue [Line Items]
Revenue									200	194	186
MIS | Financial institutions (FIG) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue									276	248	250
MIS | Public, project and infrastructure finance (PPIF)
Disaggregation of Revenue [Line Items]
Revenue									446	391	431
MIS | Public, project and infrastructure finance (PPIF) | U.S.
Disaggregation of Revenue [Line Items]
Revenue									282	229	266
MIS | Public, project and infrastructure finance (PPIF) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue									164	162	165
MIS | Rating Revenue
Disaggregation of Revenue [Line Items]
Revenue									2,846	2,693	2,755
MIS | Rating Revenue | U.S.
Disaggregation of Revenue [Line Items]
Revenue									1,720	1,618	1,701
MIS | Rating Revenue | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue									1,126	1,075	1,054
MIS | MIS Other
Disaggregation of Revenue [Line Items]
Revenue									29	19	19
MIS | MIS Other | U.S.
Disaggregation of Revenue [Line Items]
Revenue									1	1	1
MIS | MIS Other | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue									28	18	18
MA
Disaggregation of Revenue [Line Items]
Revenue									1,954	1,731	1,430
MA | U.S.
Disaggregation of Revenue [Line Items]
Revenue									823	711	646
MA | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue									1,131	1,020	784
MA | Research, data and analytics (RD&A)
Disaggregation of Revenue [Line Items]
Revenue									1,273	1,121	826
MA | Research, data and analytics (RD&A) | U.S.
Disaggregation of Revenue [Line Items]
Revenue									558	481	424
MA | Research, data and analytics (RD&A) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue									715	640	402
MA | Enterprise risk solutions (ERS)
Disaggregation of Revenue [Line Items]
Revenue									522	451	455
MA | Enterprise risk solutions (ERS) | U.S.
Disaggregation of Revenue [Line Items]
Revenue									201	170	167
MA | Enterprise risk solutions (ERS) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue									321	281	288
MA | Professional Services
Disaggregation of Revenue [Line Items]
Revenue									159	159	149
MA | Professional Services | U.S.
Disaggregation of Revenue [Line Items]
Revenue									64	60	55
MA | Professional Services | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue									$ 95	$ 99	$ 94