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Goodwill and Intangible Assets - Narrative (Details) - USD ($)
$ in Thousands
 12 Months Ended
May. 31, 2015
May. 31, 2014
May. 31, 2013
Goodwill [Line Items]      
Amortization expense of acquired intangibles $ 72,587 $ 61,945 $ 56,765
Customer-related intangible assets      
Goodwill [Line Items]      
Intangible assets weighted average amortization periods 15 years 1 month 6 days 13 years 2 months 24 days  
Contract-based intangible assets      
Goodwill [Line Items]      
Intangible assets weighted average amortization periods   13 years 2 months 18 days  
Technology-based intangible assets      
Goodwill [Line Items]      
Intangible assets weighted average amortization periods 9 years 1 month 6 days 6 years 6 months  
Trademarks and Trade Names      
Goodwill [Line Items]      
Intangible assets weighted average amortization periods 6 years 1 month 6 days