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, 2012		May 31, 2012		Feb. 29, 2012		Nov. 30, 2011		Aug. 31, 2011		May 31, 2011		Feb. 28, 2011		Nov. 30, 2010		Aug. 31, 2012	Aug. 31, 2011	Aug. 31, 2010
Quarterly Results Of Operations [Line Items]
Operating revenues	$ 127,817		$ 172,099		$ 132,134		$ 119,205		$ 116,110		$ 153,446		$ 120,168		$ 89,166		$ 551,255	$ 478,890	$ 358,440
Cost of operating revenues	95,069		123,071		95,640		88,957		86,056		111,947		86,159		64,943		402,737	349,105	259,540
Earnings before income taxes	12,653		28,587		19,427		4,441		9,777		23,159		17,001		6,577		65,108	56,514	36,782
Net earnings	$ 8,759		$ 18,823		$ 12,774		$ 2,921		$ 5,902		$ 15,289		$ 11,325		$ 4,286		$ 43,277	$ 36,802	$ 24,862
Diluted net earnings per share	$ 0.68	[1]	$ 1.47	[1]	$ 1.00	[1]	$ 0.23	[1]	$ 0.46	[1]	$ 1.20	[1]	$ 0.89	[1]	$ 0.34	[1]	$ 3.38	$ 2.90	$ 1.98
Maximum [Member]
Quarterly Results Of Operations [Line Items]
Share price	$ 74.62		$ 70.13		$ 67.27		$ 63.40		$ 73.03		$ 85.87		$ 79.56		$ 66.48		$ 74.62	$ 73.03
Minimum [Member]
Quarterly Results Of Operations [Line Items]
Share price	$ 52.68		$ 52.98		$ 49.17		$ 46.03		$ 47.68		$ 61.85		$ 58.00		$ 37.22		$ 52.68	$ 47.68

[1] The quarterly diluted net earnings per share is calculated based on the weighted average shares outstanding assuming dilution for the quarter. There may be rounding differences between the sum of the quarterly diluted net earnings per share amounts and the year-to-date diluted net earnings per share.