Revenues - Revenue by Category (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
Eliminations
Disaggregation of Revenue [Line Items]
Revenue									(143)	(136)	(128)
MIS
Disaggregation of Revenue [Line Items]
Revenue									2,875	2,712	2,774
MIS | Eliminations
Disaggregation of Revenue [Line Items]
Revenue									134	124	112
MIS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue									3,009	2,836	2,886
MIS | Corporate finance (CFG)
Disaggregation of Revenue [Line Items]
Revenue									1,497	1,379	1,448
MIS | Corporate finance (CFG) | Investment-grade
Disaggregation of Revenue [Line Items]
Revenue									379	271	335
MIS | Corporate finance (CFG) | High-yield
Disaggregation of Revenue [Line Items]
Revenue									258	175	254
MIS | Corporate finance (CFG) | Bank loans
Disaggregation of Revenue [Line Items]
Revenue									313	379	351
MIS | Corporate finance (CFG) | Other accounts
Disaggregation of Revenue [Line Items]
Revenue									547	554	508
MIS | Structured finance (SFG)
Disaggregation of Revenue [Line Items]
Revenue									427	481	440
MIS | Structured finance (SFG) | Asset-backed securities
Disaggregation of Revenue [Line Items]
Revenue									99	107	97
MIS | Structured finance (SFG) | RMBS
Disaggregation of Revenue [Line Items]
Revenue									95	98	89
MIS | Structured finance (SFG) | CMBS
Disaggregation of Revenue [Line Items]
Revenue									81	78	87
MIS | Structured finance (SFG) | Structured credit
Disaggregation of Revenue [Line Items]
Revenue									148	196	165
MIS | Structured finance (SFG) | Other accounts
Disaggregation of Revenue [Line Items]
Revenue									4	2	2
MIS | Financial institutions (FIG)
Disaggregation of Revenue [Line Items]
Revenue									476	442	436
MIS | Financial institutions (FIG) | Banking
Disaggregation of Revenue [Line Items]
Revenue									320	290	300
MIS | Financial institutions (FIG) | Insurance
Disaggregation of Revenue [Line Items]
Revenue									119	114	102
MIS | Financial institutions (FIG) | Managed investments
Disaggregation of Revenue [Line Items]
Revenue									25	25	22
MIS | Financial institutions (FIG) | Other accounts
Disaggregation of Revenue [Line Items]
Revenue									12	13	12
MIS | Public, project and infrastructure finance (PPIF)
Disaggregation of Revenue [Line Items]
Revenue									446	391	431
MIS | Public, project and infrastructure finance (PPIF) | Public finance / sovereign
Disaggregation of Revenue [Line Items]
Revenue									222	185	218
MIS | Public, project and infrastructure finance (PPIF) | Project and infrastructure
Disaggregation of Revenue [Line Items]
Revenue									224	206	213
MIS | Rating Revenue
Disaggregation of Revenue [Line Items]
Revenue									2,846	2,693	2,755
MIS | MIS Other
Disaggregation of Revenue [Line Items]
Revenue									29	19	19
MA
Disaggregation of Revenue [Line Items]
Revenue									1,954	1,731	1,430
MA | Eliminations
Disaggregation of Revenue [Line Items]
Revenue									9	12	16
MA | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue									1,963	1,743	1,446
MA | Research, data and analytics (RD&A)
Disaggregation of Revenue [Line Items]
Revenue									1,273	1,121	826
MA | Enterprise risk solutions (ERS)
Disaggregation of Revenue [Line Items]
Revenue									522	451	455
MA | Professional services (PS)
Disaggregation of Revenue [Line Items]
Revenue									$ 159	$ 159	$ 149