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, 2020
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2017
Net operating loss carryforwards          
Transition tax expense, incomplete accounting $ (500)     $ 9,400  
Provisional income tax benefit       10,500  
Cash and cash equivalents   $ 71,737 $ 105,565 349,245 $ 166,252
Income taxes paid   89,100 69,200 65,300  
Unrecognized tax benefits that, if recognized, would affect the annual effective income tax rate   1,800 3,600    
Interest and penalties included in net liability balance   100 800    
Maximum net interest expense deduction   92,000      
Interest expense   2,059 2,957 1,000  
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   $ 44,400