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Employee benefit plans and deferred compensation - Narrative (Details) - USD ($) $ in Millions | 12 Months Ended
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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 |  |

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