Property and Equipment (Details) (USD $)	May 31, 2013	May 31, 2012	May 31, 2011
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 2,342,247,000	$ 2,171,567,000
Less: accumulated depreciation	1,355,544,000	1,218,980,000
Property and equipment, net	986,703,000	952,587,000
Land [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	112,311,000	108,176,000
Buildings and improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	512,717,000	497,249,000
Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,631,213,000	1,478,270,000
Leasehold improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	27,543,000	25,502,000
Construction in progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	58,463,000	62,370,000
Capitalized interest costs [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, net	$ 0	$ 1,300,000	$ 2,200,000