Employee benefit plans and deferred compensation - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Retirement Benefits [Abstract]
Employer contribution to 401(k) plans	$ 7.0	$ 6.0	$ 4.7
Percentage of liabilities that can be met plan assets	88.00%
Investments in mutual funds within the rabbi trust	$ 15.5