Revenues - Revenue by Category (Detail) - USD ($) $ in Millions	3 Months Ended					12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 1,142.1				$ 1,126.7
Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenues	(34.7)				(34.8)
MIS [Member]
Disaggregation of Revenue [Line Items]
Revenues	670.1				719.9
MIS [Member] | Operating Segment [Member]
Disaggregation of Revenue [Line Items]
Revenues	702.4				749.7
MIS [Member] | Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenues	32.3				29.8
MIS [Member] | Corporate finance (CFG) [Member]
Disaggregation of Revenue [Line Items]
Revenues	355.4	$ 291.3	$ 307.3	$ 391.0	389.6	$ 1,379.2
MIS [Member] | Corporate finance (CFG) [Member] | Investment-grade [Member]
Disaggregation of Revenue [Line Items]
Revenues	97.4				87.2
MIS [Member] | Corporate finance (CFG) [Member] | High-yield [Member]
Disaggregation of Revenue [Line Items]
Revenues	57.3				57.9
MIS [Member] | Corporate finance (CFG) [Member] | Bank loans [Member]
Disaggregation of Revenue [Line Items]
Revenues	72.6				110.1
MIS [Member] | Corporate finance (CFG) [Member] | Other product lines [Member]
Disaggregation of Revenue [Line Items]
Revenues	128.1				134.4
MIS [Member] | Structured Finance (SFG) [Member]
Disaggregation of Revenue [Line Items]
Revenues	100.7	121.2	114.2	128.2	117.8	481.4
MIS [Member] | Structured Finance (SFG) [Member] | Assets-backed Securities [Member]
Disaggregation of Revenue [Line Items]
Revenues	23.2				28.2
MIS [Member] | Structured Finance (SFG) [Member] | RMBS [Member]
Disaggregation of Revenue [Line Items]
Revenues	23.5				24.3
MIS [Member] | Structured Finance (SFG) [Member] | CMBS [Member]
Disaggregation of Revenue [Line Items]
Revenues	17.7				21.3
MIS [Member] | Structured Finance (SFG) [Member] | Structured credit [Member]
Disaggregation of Revenue [Line Items]
Revenues	35.2				43.4
MIS [Member] | Structured Finance (SFG) [Member] | Other product lines [Member]
Disaggregation of Revenue [Line Items]
Revenues	1.1				0.6
MIS [Member] | Financial institutions (FIG) [Member]
Disaggregation of Revenue [Line Items]
Revenues	115.8				114.3
MIS [Member] | Financial institutions (FIG) [Member] | Banking [Member]
Disaggregation of Revenue [Line Items]
Revenues	79.6				77.0
MIS [Member] | Financial institutions (FIG) [Member] | Insurance [Member]
Disaggregation of Revenue [Line Items]
Revenues	29.0				28.3
MIS [Member] | Financial institutions (FIG) [Member] | Managed investments [Member]
Disaggregation of Revenue [Line Items]
Revenues	4.0				5.7
MIS [Member] | Financial institutions (FIG) [Member] | Other product lines [Member]
Disaggregation of Revenue [Line Items]
Revenues	3.2				3.3
MIS [Member] | Public, Project And Infrastructure Finance (PPIF) [Member]
Disaggregation of Revenue [Line Items]
Revenues	92.7				93.2
MIS [Member] | Public, Project And Infrastructure Finance (PPIF) [Member] | Public finance/sovereign [Member]
Disaggregation of Revenue [Line Items]
Revenues	46.2				46.9
MIS [Member] | Public, Project And Infrastructure Finance (PPIF) [Member] | Project and infrastructure [Member]
Disaggregation of Revenue [Line Items]
Revenues	46.5				46.3
MIS [Member] | Rating Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenues	664.6				714.9
MIS [Member] | MIS Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	5.5				5.0
MA [Member]
Disaggregation of Revenue [Line Items]
Revenues	472.0				406.8
MA [Member] | Operating Segment [Member]
Disaggregation of Revenue [Line Items]
Revenues	474.4				411.8
MA [Member] | Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenues	2.4				5.0
MA [Member] | Research, Data And Analytics (RD&A) [Member]
Disaggregation of Revenue [Line Items]
Revenues	307.7	297.1	280.3	275.9	267.1	1,120.4
MA [Member] | Enterprise Risk Solutions (ERS) [Member]
Disaggregation of Revenue [Line Items]
Revenues	121.9	$ 124.1	$ 115.3	$ 109.5	102.2	$ 451.1
MA [Member] | Professional Services (PS) [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 42.4				$ 37.5