Annual report pursuant to Section 13 and 15(d)

Income taxes - Narrative (Details)

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Income taxes - Narrative (Details) - USD ($)
$ in Thousands
 12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2015
Net operating loss carryforwards        
Cash and cash equivalents $ 349,245 $ 166,252 $ 66,199 $ 74,672
Income taxes paid $ 65,300 $ 87,800 $ 108,300  
Unrecognized tax benefits        
Statutory income tax rate (as a percent) 24.50% 35.00% 35.00%  
Tax Cuts and Jobs Act of 2017, incomplete accounting, change in tax rate, deferred tax liability, provisional income tax (expense) benefit $ (10,500)      
Tax Cuts and Jobs Act of 2017, incomplete accounting, transition tax for accumulated foreign earnings, provisional income tax expense (benefit) 9,400      
Unrecognized tax benefits that, if recognized, would affect the annual effective income tax rate 1,300 $ 1,100    
Interest and penalties included in net liability balance 700 700    
Maximum        
Unrecognized tax benefits        
Interest expense recognized related to unrecognized tax benefits 100 $ 100 $ 100  
Foreign Tax Authority        
Net operating loss carryforwards        
Cash and cash equivalents $ 47,100