Transaction and Relationship Revenue (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 1,126.7	$ 975.2
Percentage of Revenues	100.00%	100.00%
MIS [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 749.7	$ 694.2
MIS [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 719.9	$ 668.2
Percentage of Revenues	100.00%	100.00%
MA [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 411.8	$ 310.7
MA [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 406.8	$ 307.0
Percentage of Revenues	100.00%	100.00%
Corporate finance (CFG) [Member] | MIS [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 377.7	$ 352.8
Percentage of Revenues	100.00%	100.00%
Structured Finance (SFG) [Member] | MIS [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 129.7	$ 100.2
Percentage of Revenues	100.00%	100.00%
Financial institutions (FIG) [Member] | MIS [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 114.3	$ 112.3
Percentage of Revenues	100.00%	100.00%
Public, Project And Infrastructure Finance (PPIF) [Member] | MIS [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 93.2	$ 98.1
Percentage of Revenues	100.00%	100.00%
MIS Other [Member] | MIS [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 5.0	$ 4.8
Percentage of Revenues	100.00%	100.00%
Transaction Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 523.8	$ 495.6
Percentage of Revenues	46.00%	51.00%
Transaction Revenue [Member] | MIS [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 463.0	$ 431.0
Percentage of Revenues	64.00%	65.00%
Transaction Revenue [Member] | MA [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 60.8	$ 64.6
Percentage of Revenues	15.00%	21.00%
Transaction Revenue [Member] | Corporate finance (CFG) [Member] | MIS [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 274.9	$ 260.6
Percentage of Revenues	73.00%	74.00%
Transaction Revenue [Member] | Structured Finance (SFG) [Member] | MIS [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 83.1	$ 57.5
Percentage of Revenues	64.00%	57.00%
Transaction Revenue [Member] | Financial institutions (FIG) [Member] | MIS [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 50.0	$ 53.4
Percentage of Revenues	44.00%	48.00%
Transaction Revenue [Member] | Public, Project And Infrastructure Finance (PPIF) [Member] | MIS [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 54.4	$ 59.2
Percentage of Revenues	58.00%	60.00%
Transaction Revenue [Member] | MIS Other [Member] | MIS [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 0.6	$ 0.3
Percentage of Revenues	12.00%	6.00%
Relationship Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 602.9	$ 479.6
Percentage of Revenues	54.00%	49.00%
Relationship Revenue [Member] | MIS [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 256.9	$ 237.2
Percentage of Revenues	36.00%	35.00%
Relationship Revenue [Member] | MA [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 346.0	$ 242.4
Percentage of Revenues	85.00%	79.00%
Relationship Revenue [Member] | Corporate finance (CFG) [Member] | MIS [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 102.8	$ 92.2
Percentage of Revenues	27.00%	26.00%
Relationship Revenue [Member] | Structured Finance (SFG) [Member] | MIS [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 46.6	$ 42.7
Percentage of Revenues	36.00%	43.00%
Relationship Revenue [Member] | Financial institutions (FIG) [Member] | MIS [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 64.3	$ 58.9
Percentage of Revenues	56.00%	52.00%
Relationship Revenue [Member] | Public, Project And Infrastructure Finance (PPIF) [Member] | MIS [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 38.8	$ 38.9
Percentage of Revenues	42.00%	40.00%
Relationship Revenue [Member] | MIS Other [Member] | MIS [Member] | External Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 4.4	$ 4.5
Percentage of Revenues	88.00%	94.00%