Income Taxes - Income Tax Expense Table (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Current tax expense (benefit):
Federal									$ 14.3	$ (7.6)	$ (0.3)
State and other									2.6	0.9	0.3
Total									16.9	(6.7)	0.0
Deferred tax expense:
Federal									15.8	21.2	9.1
State and other									3.4	1.9	0.3
Total									19.2	23.1	9.4
Income tax expense	$ 14.7	$ 5.2	$ 9.2	$ 7.1	$ 7.5	$ 0.2	$ 3.3	$ 5.4	$ 36.1	$ 16.4	$ 9.4