Fee Revenue - Schedule of Disaggregation of Fee Revenue by Industry (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Oct. 31, 2018	Oct. 31, 2017	Oct. 31, 2018	Oct. 31, 2017
Disaggregation Of Revenue [Line Items]
Fee Revenue	$ 506,793	$ 455,468	$ 985,155	$ 870,385
Industrial
Disaggregation Of Revenue [Line Items]
Fee Revenue	$ 144,199	$ 132,648	$ 279,963	$ 254,095
Fee Revenue, Percentage	29.10%	29.90%	29.10%	30.10%
Financial Services
Disaggregation Of Revenue [Line Items]
Fee Revenue	$ 92,134	$ 83,467	$ 172,327	$ 150,727
Fee Revenue, Percentage	18.60%	18.80%	17.90%	17.90%
Life Sciences/ Healthcare
Disaggregation Of Revenue [Line Items]
Fee Revenue	$ 82,829	$ 71,954	$ 161,997	$ 138,534
Fee Revenue, Percentage	16.70%	16.20%	16.90%	16.40%
Consumer Goods
Disaggregation Of Revenue [Line Items]
Fee Revenue	$ 78,635	$ 70,263	$ 150,429	$ 132,094
Fee Revenue, Percentage	15.90%	15.90%	15.70%	15.60%
Technology
Disaggregation Of Revenue [Line Items]
Fee Revenue	$ 61,556	$ 56,298	$ 123,405	$ 108,070
Fee Revenue, Percentage	12.40%	12.70%	12.80%	12.80%
Education/Non-Profit
Disaggregation Of Revenue [Line Items]
Fee Revenue	$ 32,011	$ 25,842	$ 64,947	$ 54,519
Fee Revenue, Percentage	6.50%	5.80%	6.80%	6.50%
General
Disaggregation Of Revenue [Line Items]
Fee Revenue	$ 3,841	$ 2,546	$ 7,705	$ 6,233
Fee Revenue, Percentage	0.80%	0.70%	0.80%	0.70%
Fee Revenue
Disaggregation Of Revenue [Line Items]
Fee Revenue	$ 495,205	$ 443,018	$ 960,773	$ 844,272
Fee Revenue, Percentage	100.00%	100.00%	100.00%	100.00%