Retirement Plans - Defined Benefit Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit	$ 0.3	$ 0.4	$ 0.1
Expected long-term return	3.60%
Defined benefit plan contributions	$ 0.1	0.1
Defined benefit plan, expected future benefit payment, year one	2.1
Defined benefit plan, expected future benefit payment, year two	2.1
Defined benefit plan, expected future benefit payment, year three	2.2
Defined benefit plan, expected future benefit payment, year four	2.2
Defined benefit plan, expected future benefit payment, year five	2.1
Defined benefit plan, expected future benefit payment, five fiscal years thereafter	$ 10.0
Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, actual allocation, percentage	90.00%
Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, actual allocation, percentage	10.00%
Qualified Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	$ 41.5	40.8
Defined benefit plan, benefit obligation	$ 36.3	$ 38.1
Defined benefit plan, assumptions used calculating benefit obligation, discount rate	2.70%	2.55%
Defined benefit plan, funded (unfunded) status of plan	$ 5.2	$ 2.7
Nonqualified Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	0.1	0.1
Defined benefit plan, benefit obligation	1.5	1.6
Defined benefit plan, funded (unfunded) status of plan	$ (1.5)	$ (1.6)