Revenues - Transaction and Relationship Revenue (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Revenue	$ 1,600	$ 1,290
Percentage of Revenues	100.00%	100.00%
MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 1,036	$ 794
Percentage of Revenues	100.00%	100.00%
MA
Disaggregation of Revenue [Line Items]
Revenue	$ 564	$ 496
Percentage of Revenues	100.00%	100.00%
Corporate Finance (CFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 605	$ 453
Percentage of Revenues	100.00%	100.00%
Financial Institutions (FIG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 162	$ 125
Percentage of Revenues	100.00%	100.00%
Public, Project And Infrastructure Finance (PPIF) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 143	$ 109
Percentage of Revenues	100.00%	100.00%
Structured Finance (SFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 116	$ 96
Percentage of Revenues	100.00%	100.00%
MIS Other | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 10	$ 11
Percentage of Revenues	100.00%	100.00%
Research, Data And Analytics (RD&A) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 419	$ 358
Percentage of Revenues	100.00%	100.00%
Enterprise Risk Solutions (ERS) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 145	$ 138
Percentage of Revenues	100.00%	100.00%
Transaction Revenue
Disaggregation of Revenue [Line Items]
Revenue	$ 788	$ 569
Percentage of Revenues	49.00%	44.00%
Transaction Revenue | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 745	$ 519
Percentage of Revenues	72.00%	65.00%
Transaction Revenue | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 43	$ 50
Percentage of Revenues	8.00%	10.00%
Transaction Revenue | Corporate Finance (CFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 487	$ 338
Percentage of Revenues	80.00%	75.00%
Transaction Revenue | Financial Institutions (FIG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 90	$ 60
Percentage of Revenues	56.00%	48.00%
Transaction Revenue | Public, Project And Infrastructure Finance (PPIF) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 100	$ 69
Percentage of Revenues	70.00%	63.00%
Transaction Revenue | Structured Finance (SFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 66	$ 50
Percentage of Revenues	57.00%	52.00%
Transaction Revenue | MIS Other | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 2	$ 2
Percentage of Revenues	20.00%	18.00%
Transaction Revenue | Research, Data And Analytics (RD&A) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 20	$ 18
Percentage of Revenues	5.00%	5.00%
Transaction Revenue | Enterprise Risk Solutions (ERS) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 23	$ 32
Percentage of Revenues	16.00%	23.00%
Relationship Revenue
Disaggregation of Revenue [Line Items]
Revenue	$ 812	$ 721
Percentage of Revenues	51.00%	56.00%
Relationship Revenue | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 291	$ 275
Percentage of Revenues	28.00%	35.00%
Relationship Revenue | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 521	$ 446
Percentage of Revenues	92.00%	90.00%
Relationship Revenue | Corporate Finance (CFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 118	$ 115
Percentage of Revenues	20.00%	25.00%
Relationship Revenue | Financial Institutions (FIG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 72	$ 65
Percentage of Revenues	44.00%	52.00%
Relationship Revenue | Public, Project And Infrastructure Finance (PPIF) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 43	$ 40
Percentage of Revenues	30.00%	37.00%
Relationship Revenue | Structured Finance (SFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 50	$ 46
Percentage of Revenues	43.00%	48.00%
Relationship Revenue | MIS Other | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 8	$ 9
Percentage of Revenues	80.00%	82.00%
Relationship Revenue | Research, Data And Analytics (RD&A) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 399	$ 340
Percentage of Revenues	95.00%	95.00%
Relationship Revenue | Enterprise Risk Solutions (ERS) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 122	$ 106
Percentage of Revenues	84.00%	77.00%