Industry Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended			9 Months Ended
	May 31, 2012	Nov. 30, 2011	May 31, 2011	May 31, 2012	May 31, 2011	Aug. 31, 2011
Segment Reporting Information [Line Items]
Total operating revenues	$ 172,099		$ 153,446	$ 423,438	$ 362,780
Segment operating income	32,675		26,665	63,347	56,857
Unallocated general and administrative expenses	(3,888)		(3,524)	(10,529)	(10,045)
Interest and other income (expense), net	(200)		18	(363)	(75)
Earnings before income taxes	28,587		23,159	52,455	46,737
Capital Expenditures	2,850		913	7,573	5,315
Depreciation and Amortization	3,149		2,940	9,384	8,820
Assets	416,948		377,806	416,948	377,806	381,144
Environmental remediation expenses		7,200	462	7,225	1,175
Irrigation [Member]
Segment Reporting Information [Line Items]
Operating revenues	149,624		126,903	367,388	278,570
Segment operating income	31,242		25,584	64,046	47,490
Capital Expenditures	2,662		751	6,493	2,789
Depreciation and Amortization	1,710		1,329	5,055	4,069
Assets	304,515		268,283	304,515	268,283	267,275
Environmental remediation expenses			393	6,141	999
Infrastructure [Member]
Segment Reporting Information [Line Items]
Operating revenues	22,475		26,543	56,050	84,210
Segment operating income	1,433		1,081	(699)	9,367
Capital Expenditures	188		162	1,080	2,526
Depreciation and Amortization	1,439		1,611	4,329	4,751
Assets	112,433		109,523	112,433	109,523	113,869
Environmental remediation expenses			$ 69	$ 1,084	$ 176