Revenues - Revenues disaggregated by Line of Business and Geographical Area (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
U S
Disaggregation of Revenue [Line Items]
Revenues	612.1				597.7
Non Us [Member]
Disaggregation of Revenue [Line Items]
Revenues	530.0				529.0
MIS [Member]
Disaggregation of Revenue [Line Items]
Revenues	670.1				719.9
MIS [Member] | U S
Disaggregation of Revenue [Line Items]
Revenues	411.2				433.4
MIS [Member] | Non Us [Member]
Disaggregation of Revenue [Line Items]
Revenues	258.9				286.5
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] | U S
Disaggregation of Revenue [Line Items]
Revenues	242.6				257.3
MIS [Member] | Corporate finance (CFG) [Member] | Non Us [Member]
Disaggregation of Revenue [Line Items]
Revenues	112.8				132.3
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] | U S
Disaggregation of Revenue [Line Items]
Revenues	62.2				74.0
MIS [Member] | Structured Finance (SFG) [Member] | Non Us [Member]
Disaggregation of Revenue [Line Items]
Revenues	38.5				43.8
MIS [Member] | Financial institutions (FIG) [Member]
Disaggregation of Revenue [Line Items]
Revenues	115.8				114.3
MIS [Member] | Financial institutions (FIG) [Member] | U S
Disaggregation of Revenue [Line Items]
Revenues	46.0				48.5
MIS [Member] | Financial institutions (FIG) [Member] | Non Us [Member]
Disaggregation of Revenue [Line Items]
Revenues	69.8				65.8
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] | U S
Disaggregation of Revenue [Line Items]
Revenues	60.2				53.4
MIS [Member] | Public, Project And Infrastructure Finance (PPIF) [Member] | Non Us [Member]
Disaggregation of Revenue [Line Items]
Revenues	32.5				39.8
MIS [Member] | Rating Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenues	664.6				714.9
MIS [Member] | Rating Revenue [Member] | U S
Disaggregation of Revenue [Line Items]
Revenues	411.0				433.2
MIS [Member] | Rating Revenue [Member] | Non Us [Member]
Disaggregation of Revenue [Line Items]
Revenues	253.6				281.7
MIS [Member] | MIS Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	5.5				5.0
MIS [Member] | MIS Other [Member] | U S
Disaggregation of Revenue [Line Items]
Revenues	0.2				0.2
MIS [Member] | MIS Other [Member] | Non Us [Member]
Disaggregation of Revenue [Line Items]
Revenues	5.3				4.8
MA [Member]
Disaggregation of Revenue [Line Items]
Revenues	472.0				406.8
MA [Member] | U S
Disaggregation of Revenue [Line Items]
Revenues	200.9				164.3
MA [Member] | Non Us [Member]
Disaggregation of Revenue [Line Items]
Revenues	271.1				242.5
MA [Member] | Research Data And Analytics [Member]
Disaggregation of Revenue [Line Items]
Revenues	307.7	297.1	280.3	275.9	267.1	1,120.4
MA [Member] | Research Data And Analytics [Member] | U S
Disaggregation of Revenue [Line Items]
Revenues	134.8				112.6
MA [Member] | Research Data And Analytics [Member] | Non Us [Member]
Disaggregation of Revenue [Line Items]
Revenues	172.9				154.5
MA [Member] | Enterprise Risk Solutions [Member]
Disaggregation of Revenue [Line Items]
Revenues	121.9	$ 124.1	$ 115.3	$ 109.5	102.2	$ 451.1
MA [Member] | Enterprise Risk Solutions [Member] | U S
Disaggregation of Revenue [Line Items]
Revenues	48.4				38.5
MA [Member] | Enterprise Risk Solutions [Member] | Non Us [Member]
Disaggregation of Revenue [Line Items]
Revenues	73.5				63.7
MA [Member] | Professional Services [Member]
Disaggregation of Revenue [Line Items]
Revenues	42.4				37.5
MA [Member] | Professional Services [Member] | U S
Disaggregation of Revenue [Line Items]
Revenues	17.7				13.2
MA [Member] | Professional Services [Member] | Non Us [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 24.7				$ 24.3