Earnings per share (Narrative) (Details) - USD ($) shares in Thousands, $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Deferred compensation plan	$ 0.2	$ 0.2	$ 12.0
Shares reserved for issuance (in shares)	85
Performance Based Restricted Stock
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Performance based restricted stock equivalents excluded from computation (in shares)	1,100	900	500