Income Taxes - Income Tax Expense Table (Detail) - USD ($) $ in Millions	3 Months Ended								9 Months Ended		12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2020	Sep. 30, 2019
Current tax provision:
U.S. Federal									$ (0.5)		$ (7.6)	$ (0.3)
State and other									0.0		0.9	0.3
Total									(0.5)		(6.7)	0.0
Deferred tax provision:
U.S. Federal									(23.5)		21.2	9.1
State and other									(1.3)		1.9	0.3
Total									(24.8)		23.1	9.4
Income tax expense	$ 7.5	$ 0.2	$ 3.3	$ 5.4	$ 3.4	$ 1.5	$ 3.6	$ 1.0	$ (25.3)	$ 33.4	$ 16.4	$ 9.4