REVENUES - Transaction and Relationship Revenue (Detail) - USD ($) $ in Millions	3 Months Ended					9 Months Ended		12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenue	$ 1,240.5		$ 1,080.8			$ 3,596.2	$ 3,382.6
Percentage of Revenues	100.00%		100.00%			100.00%	100.00%
MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 746.6		$ 644.8			$ 2,155.1	$ 2,117.0
Percentage of Revenues	100.00%		100.00%			100.00%	100.00%
MA
Disaggregation of Revenue [Line Items]
Revenue	$ 493.9		$ 436.0			$ 1,441.1	$ 1,265.6
Percentage of Revenues	100.00%		100.00%			100.00%	100.00%
Corporate Finance (CFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 392.0	$ 291.3	$ 307.3	$ 391.0	$ 389.6	$ 1,134.8	$ 1,087.9	$ 1,379.2
Percentage of Revenues	100.00%		100.00%			100.00%	100.00%
Structured Finance (SFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 105.4	$ 121.2	$ 114.2	$ 128.2	$ 117.8	$ 318.2	$ 360.2	$ 481.4
Percentage of Revenues	100.00%		100.00%			100.00%	100.00%
Financial Institutions (FIG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 120.5		$ 119.5			$ 361.5	$ 354.4
Percentage of Revenues	100.00%		100.00%			100.00%	100.00%
Public, Project And Infrastructure Finance (PPIF) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 119.8		$ 99.0			$ 321.1	$ 300.3
Percentage of Revenues	100.00%		100.00%			100.00%	100.00%
MIS Other | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 8.9		$ 4.8			$ 19.5	$ 14.2
Percentage of Revenues	100.00%		100.00%			100.00%	100.00%
Transaction Revenue
Disaggregation of Revenue [Line Items]
Revenue	$ 557.1		$ 461.1			$ 1,585.3	$ 1,547.7
Percentage of Revenues	45.00%		43.00%			44.00%	46.00%
Transaction Revenue | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 478.6		$ 390.5			$ 1,366.1	$ 1,349.4
Percentage of Revenues	64.00%		61.00%			63.00%	64.00%
Transaction Revenue | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 78.5		$ 70.6			$ 219.2	$ 198.3
Percentage of Revenues	16.00%		16.00%			15.00%	16.00%
Transaction Revenue | Corporate Finance (CFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 281.8		$ 199.4			$ 807.7	$ 767.3
Percentage of Revenues	72.00%		65.00%			71.00%	71.00%
Transaction Revenue | Structured Finance (SFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 58.4		$ 73.5			$ 183.9	$ 233.3
Percentage of Revenues	55.00%		64.00%			58.00%	65.00%
Transaction Revenue | Financial Institutions (FIG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 55.7		$ 56.2			$ 164.8	$ 162.4
Percentage of Revenues	46.00%		47.00%			46.00%	46.00%
Transaction Revenue | Public, Project And Infrastructure Finance (PPIF) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 82.1		$ 60.9			$ 208.2	$ 184.9
Percentage of Revenues	69.00%		62.00%			65.00%	62.00%
Transaction Revenue | MIS Other | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 0.6		$ 0.5			$ 1.5	$ 1.5
Percentage of Revenues	7.00%		10.00%			8.00%	11.00%
Relationship Revenue
Disaggregation of Revenue [Line Items]
Revenue	$ 683.4		$ 619.7			$ 2,010.9	$ 1,834.9
Percentage of Revenues	55.00%		57.00%			56.00%	54.00%
Relationship Revenue | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 268.0		$ 254.3			$ 789.0	$ 767.6
Percentage of Revenues	36.00%		39.00%			37.00%	36.00%
Relationship Revenue | MA
Disaggregation of Revenue [Line Items]
Revenue	$ 415.4		$ 365.4			$ 1,221.9	$ 1,067.3
Percentage of Revenues	84.00%		84.00%			85.00%	84.00%
Relationship Revenue | Corporate Finance (CFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 110.2		$ 107.9			$ 327.1	$ 320.6
Percentage of Revenues	28.00%		35.00%			29.00%	29.00%
Relationship Revenue | Structured Finance (SFG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 47.0		$ 40.7			$ 134.3	$ 126.9
Percentage of Revenues	45.00%		36.00%			42.00%	35.00%
Relationship Revenue | Financial Institutions (FIG) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 64.8		$ 63.3			$ 196.7	$ 192.0
Percentage of Revenues	54.00%		53.00%			54.00%	54.00%
Relationship Revenue | Public, Project And Infrastructure Finance (PPIF) | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 37.7		$ 38.1			$ 112.9	$ 115.4
Percentage of Revenues	31.00%		38.00%			35.00%	38.00%
Relationship Revenue | MIS Other | MIS
Disaggregation of Revenue [Line Items]
Revenue	$ 8.3		$ 4.3			$ 18.0	$ 12.7
Percentage of Revenues	93.00%		90.00%			92.00%	89.00%