Revenue by Category (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 1,126.7	$ 975.2
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] | Intersegment Royality [Member]
Disaggregation of Revenue [Line Items]
Revenues	29.8	26.0
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] | External Revenues [Member] | Investment-grade [Member]
Disaggregation of Revenue [Line Items]
Revenues	81.1	72.1
MIS [Member] | Corporate finance (CFG) [Member] | External Revenues [Member] | High-yield [Member]
Disaggregation of Revenue [Line Items]
Revenues	57.9	64.2
MIS [Member] | Corporate finance (CFG) [Member] | External Revenues [Member] | Bank loans [Member]
Disaggregation of Revenue [Line Items]
Revenues	108.1	101.4
MIS [Member] | Corporate finance (CFG) [Member] | External Revenues [Member] | Other product lines [Member]
Disaggregation of Revenue [Line Items]
Revenues	130.6	115.1
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] | External Revenues [Member] | Assets Backed Securities [Member]
Disaggregation of Revenue [Line Items]
Revenues	28.2	22.7
MIS [Member] | Structured Finance (SFG) [Member] | External Revenues [Member] | Residential Mortgage Backed Securities Product [Member]
Disaggregation of Revenue [Line Items]
Revenues	24.3	20.4
MIS [Member] | Structured Finance (SFG) [Member] | External Revenues [Member] | Commercial real estate finance [Member]
Disaggregation of Revenue [Line Items]
Revenues	33.2	29.3
MIS [Member] | Structured Finance (SFG) [Member] | External Revenues [Member] | Structured credit [Member]
Disaggregation of Revenue [Line Items]
Revenues	43.4	27.4
MIS [Member] | Structured Finance (SFG) [Member] | External Revenues [Member] | Other product lines [Member]
Disaggregation of Revenue [Line Items]
Revenues	0.6	0.4
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] | External Revenues [Member] | Banking [Member]
Disaggregation of Revenue [Line Items]
Revenues	77.0	79.1
MIS [Member] | Financial institutions (FIG) [Member] | External Revenues [Member] | Insurance [Member]
Disaggregation of Revenue [Line Items]
Revenues	28.3	25.1
MIS [Member] | Financial institutions (FIG) [Member] | External Revenues [Member] | Managed investments [Member]
Disaggregation of Revenue [Line Items]
Revenues	5.7	5.1
MIS [Member] | Financial institutions (FIG) [Member] | External Revenues [Member] | Other product lines [Member]
Disaggregation of Revenue [Line Items]
Revenues	3.3	3.0
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] | External Revenues [Member] | Public finance / sovereign [Member]
Disaggregation of Revenue [Line Items]
Revenues	46.9	53.0
MIS [Member] | Public, Project And Infrastructure Finance (PPIF) [Member] | External Revenues [Member] | Project and infrastructure [Member]
Disaggregation of Revenue [Line Items]
Revenues	46.3	45.1
MIS [Member] | Rating Revenue [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	714.9	663.4
MIS [Member] | MIS Other [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	5.0	4.8
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] | Intersegment Royality [Member]
Disaggregation of Revenue [Line Items]
Revenues	5.0	3.7
MA [Member] | Research, Data And Analytics (RD&A) [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	269.2	175.4
MA [Member] | Enterprise Risk Solutions (ERS) [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	100.1	95.9
MA [Member] | Professional Services (PS) [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	37.5	35.7
Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ (34.8)	$ (29.7)