Annual report pursuant to Section 13 and 15(d)

Employee benefit plans and deferred compensation - Narrative (Details)

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Employee benefit plans and deferred compensation - Narrative (Details) - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Retirement Benefits [Abstract]      
Employer contribution to 401(k) plans $ 7.4 $ 7.0 $ 6.0
Percentage of liabilities that can be met plan assets 95.00%    
Investments in mutual funds within the rabbi trust $ 20.3