Revenues - Transaction and Relationship Revenue (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue	$ 1,435	$ 1,214	$ 2,725	$ 2,356
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 938	$ 739	$ 1,732	$ 1,409
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
MA
Disaggregation of Revenue [Line Items]
Revenue	$ 497	$ 475	$ 993	$ 947
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
Corporate Finance (CFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 572	$ 388	$ 1,025	$ 743
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
Structured Finance (SFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 81	$ 112	$ 177	$ 213
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
Financial Institutions (FIG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 142	$ 125	$ 267	$ 241
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	$ 108	$ 242	$ 201
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
MIS Other | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 10	$ 6	$ 21	$ 11
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
Research, Data And Analytics (RD&A) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 366	$ 315	$ 724	$ 623
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
Enterprise Risk Solutions (ERS) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 131	$ 117	$ 269	$ 239
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
Professional Services | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 0	$ 43	$ 0	$ 85
Percentage of Revenues	0.00%	100.00%	0.00%	100.00%
Transaction Revenue
Disaggregation of Revenue [Line Items]
Revenue	$ 706	$ 548	$ 1,275	$ 1,029
Percentage of Revenues	49.00%	45.00%	47.00%	44.00%
Transaction Revenue | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 664	$ 478	$ 1,183	$ 888
Percentage of Revenues	71.00%	65.00%	68.00%	63.00%
Transaction Revenue | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 42	$ 70	$ 92	$ 141
Percentage of Revenues	8.00%	15.00%	9.00%	15.00%
Transaction Revenue | Corporate Finance (CFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 457	$ 277	$ 795	$ 526
Percentage of Revenues	80.00%	71.00%	78.00%	71.00%
Transaction Revenue | Structured Finance (SFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 35	$ 69	$ 85	$ 126
Percentage of Revenues	43.00%	62.00%	48.00%	59.00%
Transaction Revenue | Financial Institutions (FIG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 76	$ 61	$ 136	$ 109
Percentage of Revenues	54.00%	49.00%	51.00%	45.00%
Transaction Revenue | Public, Project And Infrastructure Finance (PPIF) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 96	$ 71	$ 165	$ 126
Percentage of Revenues	72.00%	66.00%	68.00%	63.00%
Transaction Revenue | MIS Other | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 0	$ 0	$ 2	$ 1
Percentage of Revenues	0.00%	0.00%	10.00%	9.00%
Transaction Revenue | Research, Data And Analytics (RD&A) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 16	$ 4	$ 34	$ 9
Percentage of Revenues	4.00%	1.00%	5.00%	1.00%
Transaction Revenue | Enterprise Risk Solutions (ERS) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 26	$ 23	$ 58	$ 47
Percentage of Revenues	20.00%	20.00%	22.00%	20.00%
Transaction Revenue | Professional Services | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 0	$ 43	$ 0	$ 85
Percentage of Revenues	0.00%	100.00%	0.00%	100.00%
Relationship Revenue
Disaggregation of Revenue [Line Items]
Revenue	$ 729	$ 666	$ 1,450	$ 1,327
Percentage of Revenues	51.00%	55.00%	53.00%	56.00%
Relationship Revenue | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 274	$ 261	$ 549	$ 521
Percentage of Revenues	29.00%	35.00%	32.00%	37.00%
Relationship Revenue | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 455	$ 405	$ 901	$ 806
Percentage of Revenues	92.00%	85.00%	91.00%	85.00%
Relationship Revenue | Corporate Finance (CFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 115	$ 111	$ 230	$ 217
Percentage of Revenues	20.00%	29.00%	22.00%	29.00%
Relationship Revenue | Structured Finance (SFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 46	$ 43	$ 92	$ 87
Percentage of Revenues	57.00%	38.00%	52.00%	41.00%
Relationship Revenue | Financial Institutions (FIG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 66	$ 64	$ 131	$ 132
Percentage of Revenues	46.00%	51.00%	49.00%	55.00%
Relationship Revenue | Public, Project And Infrastructure Finance (PPIF) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 37	$ 37	$ 77	$ 75
Percentage of Revenues	28.00%	34.00%	32.00%	37.00%
Relationship Revenue | MIS Other | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 10	$ 6	$ 19	$ 10
Percentage of Revenues	100.00%	100.00%	90.00%	91.00%
Relationship Revenue | Research, Data And Analytics (RD&A) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 350	$ 311	$ 690	$ 614
Percentage of Revenues	96.00%	99.00%	95.00%	99.00%
Relationship Revenue | Enterprise Risk Solutions (ERS) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 105	$ 94	$ 211	$ 192
Percentage of Revenues	80.00%	80.00%	78.00%	80.00%
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%