Income Taxes - Rate (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Effective income tax rate reconciliation at federal statutory income tax rate, amount	$ 23.0	$ 34.1	$ 29.4
Effective income tax rate reconciliation, change in enacted tax rate, amount	0.0	0.0	(9.4)
Effective income tax rate reconciliation, noncontrolling interest income (loss), amount	(16.2)	0.0	0.0
Effective income tax rate reconciliation, state and local income taxes, amount	3.0	4.9	3.3
Effective income tax rate reconciliation, deduction, amount	(1.2)	0.0	0.0
Effective income tax rate reconciliation, uncertain tax positions	1.5	0.0	0.0
Effective income tax rate reconciliation, other reconciling items, amount	(0.9)	0.4	0.4
Income tax expense, total	$ 9.2	$ 39.4	$ 23.7