Segment Reporting and Related Disclosures Income Before Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	May. 31, 2015	Feb. 28, 2015	Nov. 30, 2014	Aug. 31, 2014	May. 31, 2014	Feb. 28, 2014	Nov. 30, 2013	Aug. 31, 2013	May. 31, 2015	May. 31, 2014	May. 31, 2013
Segment Reporting Information [Line Items]
Marginal contribution									$ 57,324	$ 64,886	$ 68,246
Fixed SG&A expenses									34,396	32,747	31,535
Interest income, net									332	387	402
Other income									1,390	0	0
Income before income taxes	$ 5,646	$ 3,746	$ 7,696	$ 7,562	$ 7,368	$ 7,291	$ 8,999	$ 8,868	24,650	32,526	37,113
Operating Segments [Member] | T&M equipment
Segment Reporting Information [Line Items]
Marginal contribution									50,410	58,924	62,350
Operating Segments [Member] | DP equipment
Segment Reporting Information [Line Items]
Marginal contribution									$ 6,914	$ 5,962	$ 5,896