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Commitments and contingencies - Commitments (Details)
3 Months Ended
Mar. 31, 2018
USD ($)
Mar. 31, 2017
USD ($)
Mar. 31, 2018
EUR (€)
Mar. 31, 2018
USD ($)
Revolving credit facility | RCF | Bank syndicate RCF        
Debt Instrument [Line Items]        
Revolving credit facility, maximum borrowing capacity     € 350,000,000.0 $ 431,200,000
Long-term line of credit       $ 0
Property Leases        
Debt Instrument [Line Items]        
Operating lease expenses $ 9,700,000 $ 8,800,000    
Hosting Services        
Debt Instrument [Line Items]        
Operating lease expenses $ 12,300,000 $ 13,900,000