Revenues - Revenues disaggregated by Line of Business and Geographical Area (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Revenues	$ 1,060.1	$ 1,080.8	$ 1,175.1	$ 1,126.7	$ 1,165.5	$ 1,062.9	$ 1,000.5	$ 975.2	$ 4,442.7	$ 4,204.1	$ 3,604.2
U S
Disaggregation of Revenue [Line Items]
Revenues									2,329.6	2,348.4	2,105.5
International [Member]
Disaggregation of Revenue [Line Items]
Revenues									2,113.1	1,855.7	1,498.7
MIS [Member]
Disaggregation of Revenue [Line Items]
Revenues									2,712.4	2,773.8	2,370.8
MIS [Member] | U S
Disaggregation of Revenue [Line Items]
Revenues									1,619.2	1,702.8	1,501.9
MIS [Member] | International [Member]
Disaggregation of Revenue [Line Items]
Revenues									1,093.2	1,071.0	868.9
MIS [Member] | Corporate finance (CFG) [Member]
Disaggregation of Revenue [Line Items]
Revenues									1,334.1	1,392.7	1,122.3
MIS [Member] | Corporate finance (CFG) [Member] | U S
Disaggregation of Revenue [Line Items]
Revenues									852.3	909.7	762.9
MIS [Member] | Corporate finance (CFG) [Member] | International [Member]
Disaggregation of Revenue [Line Items]
Revenues									481.8	483.0	359.4
MIS [Member] | Structured Finance (SFG) [Member]
Disaggregation of Revenue [Line Items]
Revenues									526.5	495.5	436.8
MIS [Member] | Structured Finance (SFG) [Member] | U S
Disaggregation of Revenue [Line Items]
Revenues									342.9	340.1	293.3
MIS [Member] | Structured Finance (SFG) [Member] | International [Member]
Disaggregation of Revenue [Line Items]
Revenues									183.6	155.4	143.5
MIS [Member] | Financial Institutions (FIG) [Member]
Disaggregation of Revenue [Line Items]
Revenues									441.7	435.8	368.9
MIS [Member] | Financial Institutions (FIG) [Member] | U S
Disaggregation of Revenue [Line Items]
Revenues									194.6	186.1	160.1
MIS [Member] | Financial Institutions (FIG) [Member] | International [Member]
Disaggregation of Revenue [Line Items]
Revenues									247.1	249.7	208.8
MIS [Member] | Public, Project And Infrastructure Finance (PPIF) [Member]
Disaggregation of Revenue [Line Items]
Revenues									391.1	431.3	412.2
MIS [Member] | Public, Project And Infrastructure Finance (PPIF) [Member] | U S
Disaggregation of Revenue [Line Items]
Revenues									228.8	266.4	276.2
MIS [Member] | Public, Project And Infrastructure Finance (PPIF) [Member] | International [Member]
Disaggregation of Revenue [Line Items]
Revenues									162.3	164.9	136.0
MIS [Member] | Rating Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenues									2,693.4	2,755.3	2,340.2
MIS [Member] | Rating Revenue [Member] | U S
Disaggregation of Revenue [Line Items]
Revenues									1,618.6	1,702.3	1,492.5
MIS [Member] | Rating Revenue [Member] | International [Member]
Disaggregation of Revenue [Line Items]
Revenues									1,074.8	1,053.0	847.7
MIS [Member] | MIS Other [Member]
Disaggregation of Revenue [Line Items]
Revenues									19.0	18.5	30.6
MIS [Member] | MIS Other [Member] | U S
Disaggregation of Revenue [Line Items]
Revenues									0.6	0.5	9.4
MIS [Member] | MIS Other [Member] | International [Member]
Disaggregation of Revenue [Line Items]
Revenues									18.4	18.0	21.2
MA [Member]
Disaggregation of Revenue [Line Items]
Revenues									1,730.3	1,430.3	1,233.4
MA [Member] | U S
Disaggregation of Revenue [Line Items]
Revenues									710.4	645.6	603.6
MA [Member] | International [Member]
Disaggregation of Revenue [Line Items]
Revenues									1,019.9	784.7	629.8
MA [Member] | Research, Data And Analytics (RD&A) [Member]
Disaggregation of Revenue [Line Items]
Revenues									1,134.1	832.7	667.6
MA [Member] | Research, Data And Analytics (RD&A) [Member] | U S
Disaggregation of Revenue [Line Items]
Revenues									480.4	424.4	389.3
MA [Member] | Research, Data And Analytics (RD&A) [Member] | International [Member]
Disaggregation of Revenue [Line Items]
Revenues									653.7	408.3	278.3
MA [Member] | Enterprise Risk Solutions (ERS) [Member]
Disaggregation of Revenue [Line Items]
Revenues									437.4	448.6	418.8
MA [Member] | Enterprise Risk Solutions (ERS) [Member] | U S
Disaggregation of Revenue [Line Items]
Revenues									170.0	166.6	162.9
MA [Member] | Enterprise Risk Solutions (ERS) [Member] | International [Member]
Disaggregation of Revenue [Line Items]
Revenues									267.4	282.0	255.9
MA [Member] | Professional Services (PS) [Member]
Disaggregation of Revenue [Line Items]
Revenues									158.8	149.0	147.0
MA [Member] | Professional Services (PS) [Member] | U S
Disaggregation of Revenue [Line Items]
Revenues									60.0	54.6	51.4
MA [Member] | Professional Services (PS) [Member] | International [Member]
Disaggregation of Revenue [Line Items]
Revenues									$ 98.8	$ 94.4	$ 95.6