Quarterly Results Of Operations (Schedule of Of Quarterly Results) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Aug. 31, 2017	May 31, 2017	Feb. 28, 2017	Nov. 30, 2016	Aug. 31, 2016	May 31, 2016	Feb. 29, 2016	Nov. 30, 2015	Aug. 31, 2017	Aug. 31, 2016	Aug. 31, 2015
Operating revenues	$ 131,937	$ 151,533	$ 124,125	$ 110,390	$ 132,897	$ 141,319	$ 120,573	$ 121,622	$ 517,985	$ 516,411	$ 560,181
Cost of operating revenues	94,146	105,627	91,184	82,016	92,951	99,511	88,128	87,208	372,973	367,798	403,860
Earnings (loss) before income taxes	10,547	16,197	7,636	1,335	11,020	14,065	(6,193)	10,396	35,715	29,288	46,751
Net earnings (loss)	$ 6,342	$ 10,952	$ 5,012	$ 873	$ 7,808	$ 9,644	$ (4,129)	$ 6,944	$ 23,179	$ 20,267	$ 26,309
Diluted net earnings (loss) per share	$ 0.59	$ 1.02	$ 0.47	$ 0.08	$ 0.73	$ 0.90	$ (0.37)	$ 0.62	$ 2.17	$ 1.85	$ 2.22
Environmental Change Reducing Net Earnings [Member]
Environmental expense, net of tax			$ 8,500