Revenues - Transaction and Relationship Revenue (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Revenue	$ 1,233	$ 1,240	$ 1,214	$ 1,142	$ 1,060	$ 1,081	$ 1,175	$ 1,127	$ 4,829	$ 4,443	$ 4,204
Percentage of Revenues									100.00%	100.00%	100.00%
MIS
Disaggregation of Revenue [Line Items]
Revenue									$ 2,875	$ 2,712	$ 2,774
Percentage of Revenues									100.00%	100.00%	100.00%
MA
Disaggregation of Revenue [Line Items]
Revenue									$ 1,954	$ 1,731	$ 1,430
Percentage of Revenues									100.00%	100.00%	100.00%
Corporate finance (CFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue									$ 1,497	$ 1,379	$ 1,448
Percentage of Revenues									100.00%	100.00%	100.00%
Structured finance (SFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue									$ 427	$ 481	$ 440
Percentage of Revenues									100.00%	100.00%	100.00%
Financial institutions (FIG) | MIS
Disaggregation of Revenue [Line Items]
Revenue									$ 476	$ 442	$ 436
Percentage of Revenues									100.00%	100.00%	100.00%
Public, project and infrastructure finance (PPIF) | MIS
Disaggregation of Revenue [Line Items]
Revenue									$ 446	$ 391	$ 431
Percentage of Revenues									100.00%	100.00%	100.00%
MIS Other | MIS
Disaggregation of Revenue [Line Items]
Revenue									$ 29	$ 19	$ 19
Percentage of Revenues									100.00%	100.00%	100.00%
Research, data and analytics (RD&A) | MA
Disaggregation of Revenue [Line Items]
Revenue									$ 1,273	$ 1,121	$ 826
Percentage of Revenues									100.00%	100.00%	100.00%
Enterprise risk solutions (ERS) | MA
Disaggregation of Revenue [Line Items]
Revenue									$ 522	$ 451	$ 455
Percentage of Revenues									100.00%	100.00%	100.00%
Professional Services | MA
Disaggregation of Revenue [Line Items]
Revenue									$ 159	$ 159	$ 149
Percentage of Revenues									100.00%	100.00%	100.00%
Transaction Revenue
Disaggregation of Revenue [Line Items]
Revenue									$ 2,102	$ 1,962	$ 2,119
Percentage of Revenues									44.00%	44.00%	50.00%
Transaction Revenue | MIS
Disaggregation of Revenue [Line Items]
Revenue									$ 1,809	$ 1,686	$ 1,808
Percentage of Revenues									63.00%	62.00%	65.00%
Transaction Revenue | MA
Disaggregation of Revenue [Line Items]
Revenue									$ 293	$ 276	$ 311
Percentage of Revenues									15.00%	16.00%	22.00%
Transaction Revenue | Corporate finance (CFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue									$ 1,057	$ 949	$ 1,053
Percentage of Revenues									71.00%	69.00%	73.00%
Transaction Revenue | Structured finance (SFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue									$ 246	$ 310	$ 279
Percentage of Revenues									58.00%	64.00%	63.00%
Transaction Revenue | Financial institutions (FIG) | MIS
Disaggregation of Revenue [Line Items]
Revenue									$ 212	$ 187	$ 195
Percentage of Revenues									45.00%	42.00%	45.00%
Transaction Revenue | Public, project and infrastructure finance (PPIF) | MIS
Disaggregation of Revenue [Line Items]
Revenue									$ 292	$ 238	$ 278
Percentage of Revenues									65.00%	61.00%	65.00%
Transaction Revenue | MIS Other | MIS
Disaggregation of Revenue [Line Items]
Revenue									$ 2	$ 2	$ 3
Percentage of Revenues									7.00%	11.00%	16.00%
Transaction Revenue | Research, data and analytics (RD&A) | MA
Disaggregation of Revenue [Line Items]
Revenue									$ 16	$ 18	$ 25
Percentage of Revenues									1.00%	2.00%	3.00%
Transaction Revenue | Enterprise risk solutions (ERS) | MA
Disaggregation of Revenue [Line Items]
Revenue									$ 118	$ 99	$ 137
Percentage of Revenues									23.00%	22.00%	30.00%
Transaction Revenue | Professional Services | MA
Disaggregation of Revenue [Line Items]
Revenue									$ 159	$ 159	$ 149
Percentage of Revenues									100.00%	100.00%	100.00%
Relationship Revenue
Disaggregation of Revenue [Line Items]
Revenue									$ 2,727	$ 2,481	$ 2,085
Percentage of Revenues									56.00%	56.00%	50.00%
Relationship Revenue | MIS
Disaggregation of Revenue [Line Items]
Revenue									$ 1,066	$ 1,026	$ 966
Percentage of Revenues									37.00%	38.00%	35.00%
Relationship Revenue | MA
Disaggregation of Revenue [Line Items]
Revenue									$ 1,661	$ 1,455	$ 1,119
Percentage of Revenues									85.00%	84.00%	78.00%
Relationship Revenue | Corporate finance (CFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue									$ 440	$ 430	$ 395
Percentage of Revenues									29.00%	31.00%	27.00%
Relationship Revenue | Structured finance (SFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue									$ 181	$ 171	$ 161
Percentage of Revenues									42.00%	36.00%	37.00%
Relationship Revenue | Financial institutions (FIG) | MIS
Disaggregation of Revenue [Line Items]
Revenue									$ 264	$ 255	$ 241
Percentage of Revenues									55.00%	58.00%	55.00%
Relationship Revenue | Public, project and infrastructure finance (PPIF) | MIS
Disaggregation of Revenue [Line Items]
Revenue									$ 154	$ 153	$ 153
Percentage of Revenues									35.00%	39.00%	35.00%
Relationship Revenue | MIS Other | MIS
Disaggregation of Revenue [Line Items]
Revenue									$ 27	$ 17	$ 16
Percentage of Revenues									93.00%	89.00%	84.00%
Relationship Revenue | Research, data and analytics (RD&A) | MA
Disaggregation of Revenue [Line Items]
Revenue									$ 1,257	$ 1,103	$ 801
Percentage of Revenues									99.00%	98.00%	97.00%
Relationship Revenue | Enterprise risk solutions (ERS) | MA
Disaggregation of Revenue [Line Items]
Revenue									$ 404	$ 352	$ 318
Percentage of Revenues									77.00%	78.00%	70.00%
Relationship Revenue | Professional Services | MA
Disaggregation of Revenue [Line Items]
Revenue									$ 0	$ 0	$ 0
Percentage of Revenues									0.00%	0.00%	0.00%