Revenues - Revenue by Category (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
Eliminations
Disaggregation of Revenue [Line Items]
Revenue	(44)	(39)	(130)	(115)
MIS
Disaggregation of Revenue [Line Items]
Revenue	925	825	2,941	2,557
MIS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	967	863	3,065	2,667
MIS | Eliminations
Disaggregation of Revenue [Line Items]
Revenue	42	38	124	110
MIS | Corporate Finance (CFG)
Disaggregation of Revenue [Line Items]
Revenue	488	461	1,643	1,486
MIS | Corporate Finance (CFG) | Investment-grade
Disaggregation of Revenue [Line Items]
Revenue	105	141	341	576
MIS | Corporate Finance (CFG) | High-yield
Disaggregation of Revenue [Line Items]
Revenue	82	101	347	275
MIS | Corporate Finance (CFG) | Bank loans
Disaggregation of Revenue [Line Items]
Revenue	145	73	482	205
MIS | Corporate Finance (CFG) | Other accounts
Disaggregation of Revenue [Line Items]
Revenue	156	146	473	430
MIS | Financial Institutions (FIG)
Disaggregation of Revenue [Line Items]
Revenue	153	134	465	401
MIS | Financial Institutions (FIG) | Other accounts
Disaggregation of Revenue [Line Items]
Revenue	2	2	7	7
MIS | Financial Institutions (FIG) | Banking
Disaggregation of Revenue [Line Items]
Revenue	105	95	315	269
MIS | Financial Institutions (FIG) | Insurance
Disaggregation of Revenue [Line Items]
Revenue	38	31	114	105
MIS | Financial Institutions (FIG) | Managed investments
Disaggregation of Revenue [Line Items]
Revenue	8	6	29	20
MIS | Public, Project And Infrastructure Finance (PPIF)
Disaggregation of Revenue [Line Items]
Revenue	130	133	403	375
MIS | Public, Project And Infrastructure Finance (PPIF) | Public finance / sovereign
Disaggregation of Revenue [Line Items]
Revenue	61	71	191	192
MIS | Public, Project And Infrastructure Finance (PPIF) | Project and infrastructure
Disaggregation of Revenue [Line Items]
Revenue	69	62	212	183
MIS | Structured Finance (SFG)
Disaggregation of Revenue [Line Items]
Revenue	143	88	399	265
MIS | Structured Finance (SFG) | Other accounts
Disaggregation of Revenue [Line Items]
Revenue	0	0	1	2
MIS | Structured Finance (SFG) | Asset-backed securities
Disaggregation of Revenue [Line Items]
Revenue	29	25	88	70
MIS | Structured Finance (SFG) | RMBS
Disaggregation of Revenue [Line Items]
Revenue	31	24	89	74
MIS | Structured Finance (SFG) | CMBS
Disaggregation of Revenue [Line Items]
Revenue	26	15	73	45
MIS | Structured Finance (SFG) | Structured credit
Disaggregation of Revenue [Line Items]
Revenue	57	24	148	74
MIS | Rating Revenue
Disaggregation of Revenue [Line Items]
Revenue	914	816	2,910	2,527
MIS | MIS Other
Disaggregation of Revenue [Line Items]
Revenue	11	9	31	30
MA
Disaggregation of Revenue [Line Items]
Revenue	601	531	1,738	1,524
MA | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	603	532	1,744	1,529
MA | Eliminations
Disaggregation of Revenue [Line Items]
Revenue	2	1	6	5
MA | Research, Data And Analytics (RD&A)
Disaggregation of Revenue [Line Items]
Revenue	445	386	1,299	1,110
MA | Enterprise Risk Solutions (ERS)
Disaggregation of Revenue [Line Items]
Revenue	$ 156	$ 145	$ 439	$ 414