Revenues - Transaction and Relationship Revenue (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Revenue	$ 1,553	$ 1,435	$ 3,153	$ 2,725
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 980	$ 938	$ 2,016	$ 1,732
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
MA
Disaggregation of Revenue [Line Items]
Revenue	$ 573	$ 497	$ 1,137	$ 993
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
Corporate Finance (CFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 550	$ 572	$ 1,155	$ 1,025
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
Financial Institutions (FIG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 150	$ 142	$ 312	$ 267
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
Public, Project And Infrastructure Finance (PPIF) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 130	$ 133	$ 273	$ 242
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
Structured Finance (SFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 140	$ 81	$ 256	$ 177
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
MIS Other | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 10	$ 10	$ 20	$ 21
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
Research, Data And Analytics (RD&A) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 435	$ 366	$ 854	$ 724
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
Enterprise Risk Solutions (ERS) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 138	$ 131	$ 283	$ 269
Percentage of Revenues	100.00%	100.00%	100.00%	100.00%
Transaction Revenue
Disaggregation of Revenue [Line Items]
Revenue	$ 724	$ 706	$ 1,512	$ 1,275
Percentage of Revenues	47.00%	49.00%	48.00%	47.00%
Transaction Revenue | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 686	$ 664	$ 1,431	$ 1,183
Percentage of Revenues	70.00%	71.00%	71.00%	68.00%
Transaction Revenue | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 38	$ 42	$ 81	$ 92
Percentage of Revenues	7.00%	8.00%	7.00%	9.00%
Transaction Revenue | Corporate Finance (CFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 427	$ 457	$ 914	$ 795
Percentage of Revenues	78.00%	80.00%	79.00%	78.00%
Transaction Revenue | Financial Institutions (FIG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 79	$ 76	$ 169	$ 136
Percentage of Revenues	53.00%	54.00%	54.00%	51.00%
Transaction Revenue | Public, Project And Infrastructure Finance (PPIF) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 88	$ 96	$ 188	$ 165
Percentage of Revenues	68.00%	72.00%	69.00%	68.00%
Transaction Revenue | Structured Finance (SFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 92	$ 35	$ 158	$ 85
Percentage of Revenues	66.00%	43.00%	62.00%	48.00%
Transaction Revenue | MIS Other | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 0	$ 0	$ 2	$ 2
Percentage of Revenues	0.00%	0.00%	10.00%	10.00%
Transaction Revenue | Research, Data And Analytics (RD&A) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 22	$ 16	$ 42	$ 34
Percentage of Revenues	5.00%	4.00%	5.00%	5.00%
Transaction Revenue | Enterprise Risk Solutions (ERS) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 16	$ 26	$ 39	$ 58
Percentage of Revenues	12.00%	20.00%	14.00%	22.00%
Recurring Revenue
Disaggregation of Revenue [Line Items]
Revenue	$ 829	$ 729	$ 1,641	$ 1,450
Percentage of Revenues	53.00%	51.00%	52.00%	53.00%
Recurring Revenue | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 294	$ 274	$ 585	$ 549
Percentage of Revenues	30.00%	29.00%	29.00%	32.00%
Recurring Revenue | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 535	$ 455	$ 1,056	$ 901
Percentage of Revenues	93.00%	92.00%	93.00%	91.00%
Recurring Revenue | Corporate Finance (CFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 123	$ 115	$ 241	$ 230
Percentage of Revenues	22.00%	20.00%	21.00%	22.00%
Recurring Revenue | Financial Institutions (FIG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 71	$ 66	$ 143	$ 131
Percentage of Revenues	47.00%	46.00%	46.00%	49.00%
Recurring Revenue | Public, Project And Infrastructure Finance (PPIF) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 42	$ 37	$ 85	$ 77
Percentage of Revenues	32.00%	28.00%	31.00%	32.00%
Recurring Revenue | Structured Finance (SFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 48	$ 46	$ 98	$ 92
Percentage of Revenues	34.00%	57.00%	38.00%	52.00%
Recurring Revenue | MIS Other | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 10	$ 10	$ 18	$ 19
Percentage of Revenues	100.00%	100.00%	90.00%	90.00%
Recurring Revenue | Research, Data And Analytics (RD&A) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 413	$ 350	$ 812	$ 690
Percentage of Revenues	95.00%	96.00%	95.00%	95.00%
Recurring Revenue | Enterprise Risk Solutions (ERS) | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 122	$ 105	$ 244	$ 211
Percentage of Revenues	88.00%	80.00%	86.00%	78.00%