Revenue by Line of Business (Detail) - USD ($) $ in Millions	3 Months Ended				12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Revenue	$ 865.9	$ 834.9	$ 918.1	$ 865.6	$ 3,484.5	$ 3,334.3	$ 2,972.5
MIS [Member]
Segment Reporting Information [Line Items]
Revenue					2,427.7	2,353.4	2,150.2
MIS [Member] | Corporate finance (CFG) [Member]
Segment Reporting Information [Line Items]
Revenue					1,112.7	1,109.3	996.8
MIS [Member] | Structured Finance (SFG) [Member]
Segment Reporting Information [Line Items]
Revenue					449.1	426.5	382.5
MIS [Member] | Financial Institutions (FIG) [Member]
Segment Reporting Information [Line Items]
Revenue					365.6	354.7	338.8
MIS [Member] | Public, Project And Infrastructure Finance (PPIF) [Member]
Segment Reporting Information [Line Items]
Revenue					376.4	357.3	341.3
MIS [Member] | Rating Revenue [Member]
Segment Reporting Information [Line Items]
Revenue					2,303.8	2,247.8	2,059.4
MIS [Member] | MIS Other [Member]
Segment Reporting Information [Line Items]
Revenue					30.4	18.0	12.2
MIS [Member] | External Revenues [Member]
Segment Reporting Information [Line Items]
Revenue					2,334.2	2,265.8	2,071.6
MIS [Member] | Intersegment Revenue/Royality [Member]
Segment Reporting Information [Line Items]
Revenue					93.5	87.6	78.6
MA [Member]
Segment Reporting Information [Line Items]
Revenue					1,163.4	1,081.8	913.3
MA [Member] | Research, Data And Analytics (RD&A) [Member]
Segment Reporting Information [Line Items]
Revenue					626.4	571.8	519.8
MA [Member] | Enterprise Risk Solutions (ERS) [Member]
Segment Reporting Information [Line Items]
Revenue					374.0	328.5	262.5
MA [Member] | Professional Services (PS) [Member]
Segment Reporting Information [Line Items]
Revenue					149.9	168.2	118.6
MA [Member] | External Revenues [Member]
Segment Reporting Information [Line Items]
Revenue					1,150.3	1,068.5	900.9
MA [Member] | Intersegment Revenue/Royality [Member]
Segment Reporting Information [Line Items]
Revenue					13.1	13.3	12.4
Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue					$ (106.6)	$ (100.9)	$ (91.0)