Revenue Recognition (Summary of Impacts of ASU on Condensed Consolidated Statement of Earnings) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Aug. 31, 2019	May 31, 2019	Feb. 28, 2019	Nov. 30, 2018	Aug. 31, 2018	May 31, 2018	Feb. 28, 2018	Nov. 30, 2017	Aug. 31, 2019	Aug. 31, 2018	Aug. 31, 2017
Statement of Earnings
Operating revenues	$ 101,885	$ 121,054	$ 109,182	$ 111,951	$ 123,269	$ 169,571	$ 130,339	$ 124,526	$ 444,072	$ 547,705	$ 517,985
Operating income									6,115	$ 39,012	$ 40,649
ASC Topic 606 Adjustments [Member] | ASC Topic 606 [Member]
Statement of Earnings
Operating revenues									(6,359)
Operating income									(3,410)
Balance Without Adoption of ASC Topic 606 [Member] | ASC Topic 606 [Member]
Statement of Earnings
Operating revenues									437,713
Operating income									$ 2,705