Fee Revenue - Additional Information (Details 1) $ in Millions	9 Months Ended
	Jan. 31, 2019 USD ($)
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue recognized, remaining performance obligation	$ 528.9
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2019-02-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue recognized, remaining performance obligation	$ 104.4
Revenue, remaining performance obligation, expected timing of satisfaction, period	3 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2019-05-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue recognized, remaining performance obligation	$ 241.8
Revenue, remaining performance obligation, expected timing of satisfaction, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2020-05-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue recognized, remaining performance obligation	$ 100.4
Revenue, remaining performance obligation, expected timing of satisfaction, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2021-05-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue recognized, remaining performance obligation	$ 82.3
Revenue, remaining performance obligation, expected timing of satisfaction, period
Revenue, remaining performance obligation, expected timing of satisfaction, explanation	2022 and thereafter