Revenues disaggregated by Line of Business and Geographical Area (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 1,126.7	$ 975.2
U S
Disaggregation of Revenue [Line Items]
Revenues	597.7	577.8
International [Member]
Disaggregation of Revenue [Line Items]
Revenues	529.0	397.4
MIS [Member]
Disaggregation of Revenue [Line Items]
Revenues	749.7	694.2
MIS [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	719.9	668.2
MIS [Member] | U S | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	433.4	422.5
MIS [Member] | International [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	286.5	245.7
MIS [Member] | Corporate finance (CFG) [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	377.7	352.8
MIS [Member] | Corporate finance (CFG) [Member] | U S | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	246.7	243.8
MIS [Member] | Corporate finance (CFG) [Member] | International [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	131.0	109.0
MIS [Member] | Structured Finance (SFG) [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	129.7	100.2
MIS [Member] | Structured Finance (SFG) [Member] | U S | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	84.6	65.0
MIS [Member] | Structured Finance (SFG) [Member] | International [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	45.1	35.2
MIS [Member] | Financial institutions (FIG) [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	114.3	112.3
MIS [Member] | Financial institutions (FIG) [Member] | U S | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	48.5	50.6
MIS [Member] | Financial institutions (FIG) [Member] | International [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	65.8	61.7
MIS [Member] | Public, Project And Infrastructure Finance (PPIF) [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	93.2	98.1
MIS [Member] | Public, Project And Infrastructure Finance (PPIF) [Member] | U S | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	53.4	63.0
MIS [Member] | Public, Project And Infrastructure Finance (PPIF) [Member] | International [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	39.8	35.1
MIS [Member] | Rating Revenue [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	714.9	663.4
MIS [Member] | Rating Revenue [Member] | U S | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	433.2	422.4
MIS [Member] | Rating Revenue [Member] | International [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	281.7	241.0
MIS [Member] | MIS Other [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	5.0	4.8
MIS [Member] | MIS Other [Member] | U S | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	0.2	0.1
MIS [Member] | MIS Other [Member] | International [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	4.8	4.7
MA [Member]
Disaggregation of Revenue [Line Items]
Revenues	411.8	310.7
MA [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	406.8	307.0
MA [Member] | U S | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	164.3	155.3
MA [Member] | International [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	242.5	151.7
MA [Member] | Research Data And Analytics [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	269.2	175.4
MA [Member] | Research Data And Analytics [Member] | U S | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	112.6	101.4
MA [Member] | Research Data And Analytics [Member] | International [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	156.6	74.0
MA [Member] | Enterprise Risk Solutions [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	100.1	95.9
MA [Member] | Enterprise Risk Solutions [Member] | U S | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	38.5	40.2
MA [Member] | Enterprise Risk Solutions [Member] | International [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	61.6	55.7
MA [Member] | Professional Services [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	37.5	35.7
MA [Member] | Professional Services [Member] | U S | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	13.2	13.7
MA [Member] | Professional Services [Member] | International [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 24.3	$ 22.0