Description of Business and Basis of Presentation - Adjustment to Retained Earnings (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
Error Corrections and Prior Period Adjustments Restatement [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
MIS
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Revenue									2,875	2,712	2,774
MIS | Corporate finance (CFG)
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Revenue									1,497	1,379	1,448
MIS | Corporate finance (CFG) | As previously reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Revenue										1,334	1,393
MIS | Corporate finance (CFG) | Reclassification
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Revenue										45	55
MIS | Structured finance (SFG)
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Revenue									427	481	440
MIS | Structured finance (SFG) | As previously reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Revenue										526	495
MIS | Structured finance (SFG) | Reclassification
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Revenue										(45)	(55)
MA
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Revenue									1,954	1,731	1,430
MA | Research, data and analytics (RD&A)
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Revenue									1,273	1,121	826
MA | Research, data and analytics (RD&A) | As previously reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Revenue										1,134	833
MA | Research, data and analytics (RD&A) | Reclassification
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Revenue										(13)	(7)
MA | Enterprise risk solutions (ERS)
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Revenue									$ 522	451	455
MA | Enterprise risk solutions (ERS) | As previously reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Revenue										438	448
MA | Enterprise risk solutions (ERS) | Reclassification
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Revenue										$ 13	$ 7