Revenues - Revenue by Category (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenue	$ 1,290	$ 1,142
Eliminations
Disaggregation of Revenue [Line Items]
Revenue	(39)	(34)
MIS
Disaggregation of Revenue [Line Items]
Revenue	794	670
MIS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	831	702
MIS | Eliminations
Disaggregation of Revenue [Line Items]
Revenue	37	32
MIS | Corporate Finance (CFG)
Disaggregation of Revenue [Line Items]
Revenue	453	355
MIS | Corporate Finance (CFG) | Investment-grade
Disaggregation of Revenue [Line Items]
Revenue	144	97
MIS | Corporate Finance (CFG) | High-yield
Disaggregation of Revenue [Line Items]
Revenue	75	57
MIS | Corporate Finance (CFG) | Bank loans
Disaggregation of Revenue [Line Items]
Revenue	89	73
MIS | Corporate Finance (CFG) | Other accounts
Disaggregation of Revenue [Line Items]
Revenue	145	128
MIS | Structured Finance (SFG)
Disaggregation of Revenue [Line Items]
Revenue	96	101
MIS | Structured Finance (SFG) | Other accounts
Disaggregation of Revenue [Line Items]
Revenue	1	1
MIS | Structured Finance (SFG) | Asset-backed securities
Disaggregation of Revenue [Line Items]
Revenue	22	23
MIS | Structured Finance (SFG) | RMBS
Disaggregation of Revenue [Line Items]
Revenue	27	24
MIS | Structured Finance (SFG) | CMBS
Disaggregation of Revenue [Line Items]
Revenue	17	18
MIS | Structured Finance (SFG) | Structured credit
Disaggregation of Revenue [Line Items]
Revenue	29	35
MIS | Financial Institutions (FIG)
Disaggregation of Revenue [Line Items]
Revenue	125	116
MIS | Financial Institutions (FIG) | Other accounts
Disaggregation of Revenue [Line Items]
Revenue	3	3
MIS | Financial Institutions (FIG) | Banking
Disaggregation of Revenue [Line Items]
Revenue	86	80
MIS | Financial Institutions (FIG) | Insurance
Disaggregation of Revenue [Line Items]
Revenue	30	29
MIS | Financial Institutions (FIG) | Managed investments
Disaggregation of Revenue [Line Items]
Revenue	6	4
MIS | Public, Project And Infrastructure Finance (PPIF)
Disaggregation of Revenue [Line Items]
Revenue	109	93
MIS | Public, Project And Infrastructure Finance (PPIF) | Public finance / sovereign
Disaggregation of Revenue [Line Items]
Revenue	57	46
MIS | Public, Project And Infrastructure Finance (PPIF) | Project and infrastructure
Disaggregation of Revenue [Line Items]
Revenue	52	47
MIS | Rating Revenue
Disaggregation of Revenue [Line Items]
Revenue	783	665
MIS | MIS Other
Disaggregation of Revenue [Line Items]
Revenue	11	5
MA
Disaggregation of Revenue [Line Items]
Revenue	496	472
MA | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	498	474
MA | Eliminations
Disaggregation of Revenue [Line Items]
Revenue	2	2
MA | Research, Data And Analytics (RD&A)
Disaggregation of Revenue [Line Items]
Revenue	358	308
MA | Enterprise Risk Solutions (ERS)
Disaggregation of Revenue [Line Items]
Revenue	138	122
MA | Professional Services (PS)
Disaggregation of Revenue [Line Items]
Revenue	$ 0	$ 42