Segment Reporting and Related Disclosures Depreciation (Details) - USD ($) $ in Thousands	12 Months Ended
	May. 31, 2015	May. 31, 2014	May. 31, 2013
Segment Reporting Information [Line Items]
Depreciation expense	$ 56,445	$ 57,034	$ 56,795
Operating Segments [Member] | T&M equipment
Segment Reporting Information [Line Items]
Depreciation expense	52,278	52,652	52,692
Operating Segments [Member] | DP equipment
Segment Reporting Information [Line Items]
Depreciation expense	$ 4,167	$ 4,382	$ 4,103