Revenues - Transaction and Relationship Revenue (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenue	$ 1,290	$ 1,142
Percentage of Revenues	100.00%	100.00%
MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 794	$ 670
Percentage of Revenues	100.00%	100.00%
MA
Disaggregation of Revenue [Line Items]
Revenue	$ 496	$ 472
Percentage of Revenues	100.00%	100.00%
Corporate Finance (CFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 453	$ 355
Percentage of Revenues	100.00%	100.00%
Structured Finance (SFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 96	$ 101
Percentage of Revenues	100.00%	100.00%
Financial Institutions (FIG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 125	$ 116
Percentage of Revenues	100.00%	100.00%
Public, Project And Infrastructure Finance (PPIF) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 109	$ 93
Percentage of Revenues	100.00%	100.00%
MIS Other | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 11	$ 5
Percentage of Revenues	100.00%	100.00%
Research, Data And Analytics (RD&A) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 358	$ 308
Percentage of Revenues	100.00%	100.00%
Enterprise Risk Solutions (ERS) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 138	$ 122
Percentage of Revenues	100.00%	100.00%
Professional Services | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 0	$ 42
Percentage of Revenues		100.00%
Transaction Revenue
Disaggregation of Revenue [Line Items]
Revenue	$ 569	$ 481
Percentage of Revenues	44.00%	42.00%
Transaction Revenue | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 519	$ 410
Percentage of Revenues	65.00%	61.00%
Transaction Revenue | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 50	$ 71
Percentage of Revenues	10.00%	15.00%
Transaction Revenue | Corporate Finance (CFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 338	$ 249
Percentage of Revenues	75.00%	70.00%
Transaction Revenue | Structured Finance (SFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 50	$ 57
Percentage of Revenues	52.00%	56.00%
Transaction Revenue | Financial Institutions (FIG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 60	$ 48
Percentage of Revenues	48.00%	41.00%
Transaction Revenue | Public, Project And Infrastructure Finance (PPIF) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 69	$ 55
Percentage of Revenues	63.00%	59.00%
Transaction Revenue | MIS Other | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 2	$ 1
Percentage of Revenues	18.00%	20.00%
Transaction Revenue | Research, Data And Analytics (RD&A) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 18	$ 5
Percentage of Revenues	5.00%	2.00%
Transaction Revenue | Enterprise Risk Solutions (ERS) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 32	$ 24
Percentage of Revenues	23.00%	20.00%
Transaction Revenue | Professional Services | MA
Disaggregation of Revenue [Line Items]
Revenue		$ 42
Percentage of Revenues		100.00%
Relationship Revenue
Disaggregation of Revenue [Line Items]
Revenue	$ 721	$ 661
Percentage of Revenues	56.00%	58.00%
Relationship Revenue | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 275	$ 260
Percentage of Revenues	35.00%	39.00%
Relationship Revenue | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 446	$ 401
Percentage of Revenues	90.00%	85.00%
Relationship Revenue | Corporate Finance (CFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 115	$ 106
Percentage of Revenues	25.00%	30.00%
Relationship Revenue | Structured Finance (SFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 46	$ 44
Percentage of Revenues	48.00%	44.00%
Relationship Revenue | Financial Institutions (FIG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 65	$ 68
Percentage of Revenues	52.00%	59.00%
Relationship Revenue | Public, Project And Infrastructure Finance (PPIF) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 40	$ 38
Percentage of Revenues	37.00%	41.00%
Relationship Revenue | MIS Other | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 9	$ 4
Percentage of Revenues	82.00%	80.00%
Relationship Revenue | Research, Data And Analytics (RD&A) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 340	$ 303
Percentage of Revenues	95.00%	98.00%
Relationship Revenue | Enterprise Risk Solutions (ERS) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 106	$ 98
Percentage of Revenues	77.00%	80.00%
Relationship Revenue | Professional Services | MA
Disaggregation of Revenue [Line Items]
Revenue		$ 0
Percentage of Revenues		0.00%