Quarterly Results Of Operations (Schedule of Of Quarterly Results) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Aug. 31, 2018	May 31, 2018	Feb. 28, 2018	Nov. 30, 2017	Aug. 31, 2017	May 31, 2017	Feb. 28, 2017	Nov. 30, 2016	Aug. 31, 2018	Aug. 31, 2017	Aug. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 123,269	$ 169,571	$ 130,339	$ 124,526	$ 131,937	$ 151,533	$ 124,125	$ 110,390	$ 547,705	$ 517,985	$ 516,411
Cost of operating revenues	90,998	118,093	95,023	92,129	94,146	105,627	91,184	82,016	396,243	372,973	367,798
Earnings before income taxes	5,940	17,445	5,676	4,792	10,547	16,197	7,636	1,335	33,853	35,715	29,288
Net earnings	$ 4,978	$ 10,379	$ 1,735	$ 3,185	$ 6,342	$ 10,952	$ 5,012	$ 873	$ 20,277	$ 23,179	$ 20,267
Diluted net earnings per share	$ 0.46	$ 0.96	$ 0.16	$ 0.30	$ 0.59	$ 1.02	$ 0.47	$ 0.08	$ 1.88	$ 2.17	$ 1.85