Revenues - Revenues Disaggregated by Line of Business and Geographical Area (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Disaggregation of Revenue [Line Items]
Revenue	$ 6,218	$ 5,371	$ 4,829
U.S.
Disaggregation of Revenue [Line Items]
Revenue	3,416	2,955	2,544
Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	2,802	2,416	2,285
MIS
Disaggregation of Revenue [Line Items]
Revenue	3,812	3,292	2,875
MIS | U.S.
Disaggregation of Revenue [Line Items]
Revenue	2,344	2,068	1,721
MIS | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	1,468	1,224	1,154
MIS | Rating Revenue
Disaggregation of Revenue [Line Items]
Revenue	3,770	3,245	2,846
MIS | Rating Revenue | U.S.
Disaggregation of Revenue [Line Items]
Revenue	2,341	2,066	1,720
MIS | Rating Revenue | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	1,429	1,179	1,126
MIS | Corporate finance (CFG)
Disaggregation of Revenue [Line Items]
Revenue	2,087	1,857	1,497
MIS | Corporate finance (CFG) | U.S.
Disaggregation of Revenue [Line Items]
Revenue	1,384	1,291	968
MIS | Corporate finance (CFG) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	703	566	529
MIS | Financial institutions (FIG)
Disaggregation of Revenue [Line Items]
Revenue	602	530	476
MIS | Financial institutions (FIG) | U.S.
Disaggregation of Revenue [Line Items]
Revenue	289	250	200
MIS | Financial institutions (FIG) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	313	280	276
MIS | Public, project and infrastructure finance (PPIF)
Disaggregation of Revenue [Line Items]
Revenue	521	496	446
MIS | Public, project and infrastructure finance (PPIF) | U.S.
Disaggregation of Revenue [Line Items]
Revenue	304	311	282
MIS | Public, project and infrastructure finance (PPIF) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	217	185	164
MIS | Structured finance (SFG)
Disaggregation of Revenue [Line Items]
Revenue	560	362	427
MIS | Structured finance (SFG) | U.S.
Disaggregation of Revenue [Line Items]
Revenue	364	214	270
MIS | Structured finance (SFG) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	196	148	157
MIS | MIS Other
Disaggregation of Revenue [Line Items]
Revenue	42	47	29
MIS | MIS Other | U.S.
Disaggregation of Revenue [Line Items]
Revenue	3	2	1
MIS | MIS Other | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	39	45	28
MA
Disaggregation of Revenue [Line Items]
Revenue	2,406	2,079	1,954
MA | U.S.
Disaggregation of Revenue [Line Items]
Revenue	1,072	887	823
MA | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	1,334	1,192	1,131
MA | Research, data and analytics (RD&A)
Disaggregation of Revenue [Line Items]
Revenue	1,745	1,514	1,273
MA | Research, data and analytics (RD&A) | U.S.
Disaggregation of Revenue [Line Items]
Revenue	758	668	558
MA | Research, data and analytics (RD&A) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	987	846	715
MA | Enterprise risk solutions (ERS)
Disaggregation of Revenue [Line Items]
Revenue	661	565	522
MA | Enterprise risk solutions (ERS) | U.S.
Disaggregation of Revenue [Line Items]
Revenue	314	219	201
MA | Enterprise risk solutions (ERS) | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	347	346	321
MA | Professional Services
Disaggregation of Revenue [Line Items]
Revenue	0	0	159
MA | Professional Services | U.S.
Disaggregation of Revenue [Line Items]
Revenue	0	0	64
MA | Professional Services | Non-U.S.
Disaggregation of Revenue [Line Items]
Revenue	$ 0	$ 0	$ 95