Accumulated Other Comprehensive Loss, Reclassification out of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jan. 31, 2018	Jan. 31, 2017	Jan. 31, 2018	Jan. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating and administrative expenses	$ 248,746	$ 247,278	$ 731,872	$ 729,775
Unamortized Retirement Costs [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating and administrative expenses	$ 1,500	$ 1,200	$ 4,400	$ 9,900