Revenue by Line of Business (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Revenue	$ 1,165.5	$ 1,062.9	$ 1,000.5	$ 975.2	$ 942.1	$ 917.1	$ 928.9	$ 816.1	$ 4,204.1	$ 3,604.2	$ 3,484.5
MIS [Member]
Segment Reporting Information [Line Items]
Revenue									2,885.5	2,471.0	2,427.7
MIS [Member] | Corporate finance (CFG) [Member]
Segment Reporting Information [Line Items]
Revenue									1,392.7	1,122.3	1,112.7
MIS [Member] | Structured Finance (SFG) [Member]
Segment Reporting Information [Line Items]
Revenue									495.5	436.8	449.1
MIS [Member] | Financial Institutions (FIG) [Member]
Segment Reporting Information [Line Items]
Revenue									435.8	368.9	365.6
MIS [Member] | Public, Project And Infrastructure Finance (PPIF) [Member]
Segment Reporting Information [Line Items]
Revenue									431.3	412.2	376.4
MIS [Member] | Rating Revenue [Member]
Segment Reporting Information [Line Items]
Revenue									2,755.3	2,340.2	2,303.8
MIS [Member] | MIS Other [Member]
Segment Reporting Information [Line Items]
Revenue									18.5	30.6	30.4
MIS [Member] | External Revenues [Member]
Segment Reporting Information [Line Items]
Revenue									2,773.8	2,370.8	2,334.2
MIS [Member] | Intersegment Revenue/Royality [Member]
Segment Reporting Information [Line Items]
Revenue									111.7	100.2	93.5
MA [Member]
Segment Reporting Information [Line Items]
Revenue									1,446.3	1,246.9	1,163.4
MA [Member] | Research, Data And Analytics (RD&A) [Member]
Segment Reporting Information [Line Items]
Revenue									832.7	667.6	626.4
MA [Member] | Enterprise Risk Solutions (ERS) [Member]
Segment Reporting Information [Line Items]
Revenue									448.6	418.8	374.0
MA [Member] | Professional Services (PS) [Member]
Segment Reporting Information [Line Items]
Revenue									149.0	147.0	149.9
MA [Member] | External Revenues [Member]
Segment Reporting Information [Line Items]
Revenue									1,430.3	1,233.4	1,150.3
MA [Member] | Intersegment Revenue/Royality [Member]
Segment Reporting Information [Line Items]
Revenue									16.0	13.5	13.1
Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue									$ (127.7)	$ (113.7)	$ (106.6)