Annual report pursuant to Section 13 and 15(d)

Income taxes - Narrative (Details)

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Income taxes - Narrative (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2018
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Net operating loss carryforwards          
Transition tax expense (benefit) $ (500)        
Transition tax expense, incomplete accounting     $ 9,400    
Tax Cuts and Jobs Act of 2017, incomplete accounting, change in tax rate, deferred tax liability, provisional income tax benefit     10,500    
Cash and cash equivalents   $ 105,565 349,245 $ 166,252 $ 66,199
Income taxes paid   69,200 65,300 87,800  
Unrecognized tax benefits that, if recognized, would affect the annual effective income tax rate   3,600 1,300    
Interest and penalties included in net liability balance   800 800    
Maximum          
Net operating loss carryforwards          
Interest expense recognized related to unrecognized tax benefits (less than)   100 $ 100 $ 100  
Foreign Tax Authority          
Net operating loss carryforwards          
Cash and cash equivalents   $ 20,300