Quarterly Results Of Operations (Schedule of Of Quarterly Results) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Aug. 31, 2013	May 31, 2013	Feb. 28, 2013	Nov. 30, 2012	Aug. 31, 2012	May 31, 2012	Feb. 29, 2012	Nov. 30, 2011	Aug. 31, 2013	Aug. 31, 2012	Aug. 31, 2011
Quarterly Results Of Operations [Abstract]
Operating revenues	$ 148,397	$ 219,542	$ 175,539	$ 147,370	$ 127,817	$ 172,099	$ 132,134	$ 119,205	$ 690,848	$ 551,255	$ 478,890
Cost of operating revenues	109,820	156,506	125,175	104,513	95,069	123,071	95,640	88,957	496,014	402,737	349,105
Earnings before income taxes	15,686	39,750	29,488	22,383	12,653	28,587	19,427	4,441	107,307	65,108	56,514
Net earnings	$ 10,428	$ 26,063	$ 19,351	$ 14,728	$ 8,759	$ 18,823	$ 12,774	$ 2,921	$ 70,570	$ 43,277	$ 36,802
Diluted net earnings per share	$ 0.81	$ 2.01	$ 1.50	$ 1.15	$ 0.68	$ 1.47	$ 1.00	$ 0.23	$ 5.47	$ 3.38	$ 2.90