Revenues - Expected Recognition Period for Remaining Performance Obligations (Detail) $ in Millions	Mar. 31, 2019 USD ($)
MIS [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-03-31
Disaggregation of Revenue [Line Items]
Remaining Performance Obligation	$ 151.2
Remaining Performance Obligation, Expected Timing of Satisfaction, Period	0 years
MIS [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-04-01
Disaggregation of Revenue [Line Items]
Remaining Performance Obligation	$ 23.5
Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year
MIS [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-04-01
Disaggregation of Revenue [Line Items]
Remaining Performance Obligation	$ 70.2
Remaining Performance Obligation, Expected Timing of Satisfaction, Period	4 years
MIS [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-04-01
Disaggregation of Revenue [Line Items]
Remaining Performance Obligation	$ 41.5
Remaining Performance Obligation, Expected Timing of Satisfaction, Period	4 years
MIS [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2029-04-01
Disaggregation of Revenue [Line Items]
Remaining Performance Obligation	$ 6.5
Remaining Performance Obligation, Expected Timing of Satisfaction, Period	4 years
MIS [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2034-04-01
Disaggregation of Revenue [Line Items]
Remaining Performance Obligation	$ 4.1
Remaining Performance Obligation, Expected Timing of Satisfaction, Period	4 years
MIS [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2039-04-01
Disaggregation of Revenue [Line Items]
Remaining Performance Obligation	$ 5.4
Remaining Performance Obligation, Expected Timing of Satisfaction, Period	4 years
MA [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-03-31
Disaggregation of Revenue [Line Items]
Remaining Performance Obligation	$ 2,053.3
Remaining Performance Obligation, Expected Timing of Satisfaction, Period	0 years
MA [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-04-01
Disaggregation of Revenue [Line Items]
Remaining Performance Obligation	$ 1,334.0
Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year
MA [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-04-01
Disaggregation of Revenue [Line Items]
Remaining Performance Obligation	$ 491.0
Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year
MA [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-03-31
Disaggregation of Revenue [Line Items]
Remaining Performance Obligation	$ 228.3
Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year