Expected Recognition Period for Remaining Performance Obligations (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
MIS [Member]
Disaggregation of Revenue [Line Items]
Remaining Performance Obligation	$ 150.5
MIS [Member] | Less Than One Year [Member]
Disaggregation of Revenue [Line Items]
Remaining Performance Obligation	$ 21.9
Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year
MIS [Member] | 1 - 5 years [Member]
Disaggregation of Revenue [Line Items]
Remaining Performance Obligation	$ 65.2
MIS [Member] | 1 - 5 years [Member] | Minimum [Member]
Disaggregation of Revenue [Line Items]
Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year
MIS [Member] | 1 - 5 years [Member] | Maximum [Member]
Disaggregation of Revenue [Line Items]
Remaining Performance Obligation, Expected Timing of Satisfaction, Period	5 years
MIS [Member] | 6 - 10 Years [Member]
Disaggregation of Revenue [Line Items]
Remaining Performance Obligation	$ 43.6
MIS [Member] | 6 - 10 Years [Member] | Minimum [Member]
Disaggregation of Revenue [Line Items]
Remaining Performance Obligation, Expected Timing of Satisfaction, Period	6 years
MIS [Member] | 6 - 10 Years [Member] | Maximum [Member]
Disaggregation of Revenue [Line Items]
Remaining Performance Obligation, Expected Timing of Satisfaction, Period	10 years
MIS [Member] | 11 - 15 years [Member]
Disaggregation of Revenue [Line Items]
Remaining Performance Obligation	$ 8.7
MIS [Member] | 11 - 15 years [Member] | Minimum [Member]
Disaggregation of Revenue [Line Items]
Remaining Performance Obligation, Expected Timing of Satisfaction, Period	11 years
MIS [Member] | 11 - 15 years [Member] | Maximum [Member]
Disaggregation of Revenue [Line Items]
Remaining Performance Obligation, Expected Timing of Satisfaction, Period	15 years
MIS [Member] | 16-20 years [Member]
Disaggregation of Revenue [Line Items]
Remaining Performance Obligation	$ 4.5
MIS [Member] | 16-20 years [Member] | Minimum [Member]
Disaggregation of Revenue [Line Items]
Remaining Performance Obligation, Expected Timing of Satisfaction, Period	16 years
MIS [Member] | 16-20 years [Member] | Maximum [Member]
Disaggregation of Revenue [Line Items]
Remaining Performance Obligation, Expected Timing of Satisfaction, Period	20 years
MIS [Member] | Over 20 Years [Member]
Disaggregation of Revenue [Line Items]
Remaining Performance Obligation	$ 6.6
Remaining Performance Obligation, Expected Timing of Satisfaction, Period	20 years
MA [Member]
Disaggregation of Revenue [Line Items]
Remaining Performance Obligation	$ 1,418.6
MA [Member] | Less Than One Year [Member]
Disaggregation of Revenue [Line Items]
Remaining Performance Obligation	$ 1,089.0
Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year
MA [Member] | 1 - 2 Years [Member]
Disaggregation of Revenue [Line Items]
Remaining Performance Obligation	$ 217.6
MA [Member] | 1 - 2 Years [Member] | Minimum [Member]
Disaggregation of Revenue [Line Items]
Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year
MA [Member] | 1 - 2 Years [Member] | Maximum [Member]
Disaggregation of Revenue [Line Items]
Remaining Performance Obligation, Expected Timing of Satisfaction, Period	2 years
MA [Member] | Over 2 Years [Member]
Disaggregation of Revenue [Line Items]
Remaining Performance Obligation	$ 112.0
Remaining Performance Obligation, Expected Timing of Satisfaction, Period	2 years