Revenues - Transaction and Relationship Revenue (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
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 825	$ 746	$ 2,557	$ 2,155
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
MA
Disaggregation of Revenue [Line Items]
Revenue	$ 531	$ 494	$ 1,524	$ 1,441
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
Corporate Finance (CFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 461	$ 392	$ 1,486	$ 1,135
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
Structured Finance (SFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 88	$ 105	$ 265	$ 318
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
Financial Institutions (FIG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 134	$ 120	$ 401	$ 361
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
Public, Project And Infrastructure Finance (PPIF) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 133	$ 120	$ 375	$ 321
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
MIS Other | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 9	$ 9	$ 30	$ 20
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
Research, Data And Analytics (RD&A) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 386	$ 317	$ 1,110	$ 940
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
Enterprise Risk Solutions (ERS) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 145	$ 134	$ 414	$ 373
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
Professional Services | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 0	$ 43	$ 0	$ 128
Percentage of Revenues	0.00%	100.00%	0.00%	100.00%
Transaction Revenue
Disaggregation of Revenue [Line Items]
Revenue	$ 599	$ 556	$ 1,874	$ 1,585
Percentage of Revenues	44.00%	45.00%	46.00%	44.00%
Transaction Revenue | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 548	$ 478	$ 1,731	$ 1,366
Percentage of Revenues	66.00%	64.00%	68.00%	63.00%
Transaction Revenue | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 51	$ 78	$ 143	$ 219
Percentage of Revenues	10.00%	16.00%	9.00%	15.00%
Transaction Revenue | Corporate Finance (CFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 347	$ 282	$ 1,142	$ 808
Percentage of Revenues	75.00%	72.00%	77.00%	71.00%
Transaction Revenue | Structured Finance (SFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 41	$ 58	$ 126	$ 184
Percentage of Revenues	47.00%	55.00%	48.00%	58.00%
Transaction Revenue | Financial Institutions (FIG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 67	$ 56	$ 203	$ 165
Percentage of Revenues	50.00%	47.00%	51.00%	46.00%
Transaction Revenue | Public, Project And Infrastructure Finance (PPIF) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 92	$ 82	$ 257	$ 208
Percentage of Revenues	69.00%	68.00%	69.00%	65.00%
Transaction Revenue | MIS Other | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 1	$ 0	$ 3	$ 1
Percentage of Revenues	11.00%	0.00%	10.00%	5.00%
Transaction Revenue | Research, Data And Analytics (RD&A) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 19	$ 3	$ 53	$ 12
Percentage of Revenues	5.00%	1.00%	5.00%	1.00%
Transaction Revenue | Enterprise Risk Solutions (ERS) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 32	$ 32	$ 90	$ 79
Percentage of Revenues	22.00%	24.00%	22.00%	21.00%
Transaction Revenue | Professional Services | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 0	$ 43	$ 0	$ 128
Percentage of Revenues	0.00%	100.00%	0.00%	100.00%
Relationship Revenue
Disaggregation of Revenue [Line Items]
Revenue	$ 757	$ 684	$ 2,207	$ 2,011
Percentage of Revenues	56.00%	55.00%	54.00%	56.00%
Relationship Revenue | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 277	$ 268	$ 826	$ 789
Percentage of Revenues	34.00%	36.00%	32.00%	37.00%
Relationship Revenue | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 480	$ 416	$ 1,381	$ 1,222
Percentage of Revenues	90.00%	84.00%	91.00%	85.00%
Relationship Revenue | Corporate Finance (CFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 114	$ 110	$ 344	$ 327
Percentage of Revenues	25.00%	28.00%	23.00%	29.00%
Relationship Revenue | Structured Finance (SFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 47	$ 47	$ 139	$ 134
Percentage of Revenues	53.00%	45.00%	52.00%	42.00%
Relationship Revenue | Financial Institutions (FIG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 67	$ 64	$ 198	$ 196
Percentage of Revenues	50.00%	53.00%	49.00%	54.00%
Relationship Revenue | Public, Project And Infrastructure Finance (PPIF) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 41	$ 38	$ 118	$ 113
Percentage of Revenues	31.00%	32.00%	31.00%	35.00%
Relationship Revenue | MIS Other | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 8	$ 9	$ 27	$ 19
Percentage of Revenues	89.00%	100.00%	90.00%	95.00%
Relationship Revenue | Research, Data And Analytics (RD&A) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 367	$ 314	$ 1,057	$ 928
Percentage of Revenues	95.00%	99.00%	95.00%	99.00%
Relationship Revenue | Enterprise Risk Solutions (ERS) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 113	$ 102	$ 324	$ 294
Percentage of Revenues	78.00%	76.00%	78.00%	79.00%
Relationship Revenue | Professional Services | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 0	$ 0	$ 0	$ 0
Percentage of Revenues	0.00%	0.00%	0.00%	0.00%