Revenues - Transaction and Relationship Revenue (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
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 925	$ 825	$ 2,941	$ 2,557
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
MA
Disaggregation of Revenue [Line Items]
Revenue	$ 601	$ 531	$ 1,738	$ 1,524
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
Corporate Finance (CFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 488	$ 461	$ 1,643	$ 1,486
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
Financial Institutions (FIG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 153	$ 134	$ 465	$ 401
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
Public, Project And Infrastructure Finance (PPIF) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 130	$ 133	$ 403	$ 375
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
Structured Finance (SFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 143	$ 88	$ 399	$ 265
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
MIS Other | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 11	$ 9	$ 31	$ 30
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
Research, Data And Analytics (RD&A) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 445	$ 386	$ 1,299	$ 1,110
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
Enterprise Risk Solutions (ERS) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 156	$ 145	$ 439	$ 414
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
Transaction Revenue
Disaggregation of Revenue [Line Items]
Revenue	$ 667	$ 599	$ 2,179	$ 1,874
Percentage of Revenues	44.00%	44.00%	47.00%	46.00%
Transaction Revenue | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 631	$ 548	$ 2,062	$ 1,731
Percentage of Revenues	68.00%	66.00%	70.00%	68.00%
Transaction Revenue | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 36	$ 51	$ 117	$ 143
Percentage of Revenues	6.00%	10.00%	7.00%	9.00%
Transaction Revenue | Corporate Finance (CFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 366	$ 347	$ 1,280	$ 1,142
Percentage of Revenues	75.00%	75.00%	78.00%	77.00%
Transaction Revenue | Financial Institutions (FIG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 83	$ 67	$ 252	$ 203
Percentage of Revenues	54.00%	50.00%	54.00%	51.00%
Transaction Revenue | Public, Project And Infrastructure Finance (PPIF) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 88	$ 92	$ 276	$ 257
Percentage of Revenues	68.00%	69.00%	68.00%	69.00%
Transaction Revenue | Structured Finance (SFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 93	$ 41	$ 251	$ 126
Percentage of Revenues	65.00%	47.00%	63.00%	48.00%
Transaction Revenue | MIS Other | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 1	$ 1	$ 3	$ 3
Percentage of Revenues	9.00%	11.00%	10.00%	10.00%
Transaction Revenue | Research, Data And Analytics (RD&A) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 21	$ 19	$ 63	$ 53
Percentage of Revenues	5.00%	5.00%	5.00%	5.00%
Transaction Revenue | Enterprise Risk Solutions (ERS) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 15	$ 32	$ 54	$ 90
Percentage of Revenues	10.00%	22.00%	12.00%	22.00%
Recurring Revenue
Disaggregation of Revenue [Line Items]
Revenue	$ 859	$ 757	$ 2,500	$ 2,207
Percentage of Revenues	56.00%	56.00%	53.00%	54.00%
Recurring Revenue | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 294	$ 277	$ 879	$ 826
Percentage of Revenues	32.00%	34.00%	30.00%	32.00%
Recurring Revenue | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 565	$ 480	$ 1,621	$ 1,381
Percentage of Revenues	94.00%	90.00%	93.00%	91.00%
Recurring Revenue | Corporate Finance (CFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 122	$ 114	$ 363	$ 344
Percentage of Revenues	25.00%	25.00%	22.00%	23.00%
Recurring Revenue | Financial Institutions (FIG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 70	$ 67	$ 213	$ 198
Percentage of Revenues	46.00%	50.00%	46.00%	49.00%
Recurring Revenue | Public, Project And Infrastructure Finance (PPIF) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 42	$ 41	$ 127	$ 118
Percentage of Revenues	32.00%	31.00%	32.00%	31.00%
Recurring Revenue | Structured Finance (SFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 50	$ 47	$ 148	$ 139
Percentage of Revenues	35.00%	53.00%	37.00%	52.00%
Recurring Revenue | MIS Other | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 10	$ 8	$ 28	$ 27
Percentage of Revenues	91.00%	89.00%	90.00%	90.00%
Recurring Revenue | Research, Data And Analytics (RD&A) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 424	$ 367	$ 1,236	$ 1,057
Percentage of Revenues	95.00%	95.00%	95.00%	95.00%
Recurring Revenue | Enterprise Risk Solutions (ERS) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 141	$ 113	$ 385	$ 324
Percentage of Revenues	90.00%	78.00%	88.00%	78.00%