Revenues - Revenue by Category (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue	$ 1,356	$ 1,240	$ 4,081	$ 3,596
Eliminations
Disaggregation of Revenue [Line Items]
Revenue	(39)	(37)	(115)	(107)
MIS
Disaggregation of Revenue [Line Items]
Revenue	825	746	2,557	2,155
MIS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	863	781	2,667	2,255
MIS | Eliminations
Disaggregation of Revenue [Line Items]
Revenue	38	35	110	100
MIS | Corporate Finance (CFG)
Disaggregation of Revenue [Line Items]
Revenue	461	392	1,486	1,135
MIS | Corporate Finance (CFG) | Investment-grade
Disaggregation of Revenue [Line Items]
Revenue	141	106	576	299
MIS | Corporate Finance (CFG) | High-yield
Disaggregation of Revenue [Line Items]
Revenue	101	56	275	182
MIS | Corporate Finance (CFG) | Bank loans
Disaggregation of Revenue [Line Items]
Revenue	73	90	205	246
MIS | Corporate Finance (CFG) | Other accounts
Disaggregation of Revenue [Line Items]
Revenue	146	140	430	408
MIS | Structured Finance (SFG)
Disaggregation of Revenue [Line Items]
Revenue	88	105	265	318
MIS | Structured Finance (SFG) | Other accounts
Disaggregation of Revenue [Line Items]
Revenue	0	1	2	3
MIS | Structured Finance (SFG) | Asset-backed securities
Disaggregation of Revenue [Line Items]
Revenue	25	25	70	74
MIS | Structured Finance (SFG) | RMBS
Disaggregation of Revenue [Line Items]
Revenue	24	21	74	69
MIS | Structured Finance (SFG) | CMBS
Disaggregation of Revenue [Line Items]
Revenue	15	18	45	56
MIS | Structured Finance (SFG) | Structured credit
Disaggregation of Revenue [Line Items]
Revenue	24	40	74	116
MIS | Financial Institutions (FIG)
Disaggregation of Revenue [Line Items]
Revenue	134	120	401	361
MIS | Financial Institutions (FIG) | Other accounts
Disaggregation of Revenue [Line Items]
Revenue	2	3	7	9
MIS | Financial Institutions (FIG) | Banking
Disaggregation of Revenue [Line Items]
Revenue	95	80	269	244
MIS | Financial Institutions (FIG) | Insurance
Disaggregation of Revenue [Line Items]
Revenue	31	31	105	88
MIS | Financial Institutions (FIG) | Managed investments
Disaggregation of Revenue [Line Items]
Revenue	6	6	20	20
MIS | Public, Project And Infrastructure Finance (PPIF)
Disaggregation of Revenue [Line Items]
Revenue	133	120	375	321
MIS | Public, Project And Infrastructure Finance (PPIF) | Public finance / sovereign
Disaggregation of Revenue [Line Items]
Revenue	71	58	192	157
MIS | Public, Project And Infrastructure Finance (PPIF) | Project and infrastructure
Disaggregation of Revenue [Line Items]
Revenue	62	62	183	164
MIS | Rating Revenue
Disaggregation of Revenue [Line Items]
Revenue	816	737	2,527	2,135
MIS | MIS Other
Disaggregation of Revenue [Line Items]
Revenue	9	9	30	20
MA
Disaggregation of Revenue [Line Items]
Revenue	531	494	1,524	1,441
MA | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	532	496	1,529	1,448
MA | Eliminations
Disaggregation of Revenue [Line Items]
Revenue	1	2	5	7
MA | Research, Data And Analytics (RD&A)
Disaggregation of Revenue [Line Items]
Revenue	386	317	1,110	940
MA | Enterprise Risk Solutions (ERS)
Disaggregation of Revenue [Line Items]
Revenue	145	134	414	373
MA | Professional Services (PS)
Disaggregation of Revenue [Line Items]
Revenue	$ 0	$ 43	$ 0	$ 128