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Accumulated Other Comprehensive Loss, Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Oct. 31, 2017
Oct. 31, 2016
Oct. 31, 2017
Oct. 31, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Operating and administrative expenses $ 239,318 $ 247,270 $ 483,126 $ 482,497
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 $ 10,000 $ 2,900 $ 8,700