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Accumulated Other Comprehensive Loss, Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($)
$ in Thousands
12 Months Ended
Apr. 30, 2018
Apr. 30, 2017
Apr. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]      
Operating and administrative expenses $ 994,552 $ 988,597 $ 994,372
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 $ 5,900 $ 11,100