Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended		3 Months Ended		6 Months Ended
	Nov. 30, 2018	Nov. 30, 2013	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Significant Accounting Policies [Line Items]
Net sales and other operating revenues			$ 844.0	$ 818.0	$ 1,665.0	$ 1,538.0
Cost of sales			666.0	$ 630.0	1,321.0	$ 1,174.0
Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Significant Accounting Policies [Line Items]
Net sales and other operating revenues			(20.0)		(40.0)
Cost of sales			$ (20.0)		$ (40.0)
NHUMO [Member]
Significant Accounting Policies [Line Items]
Preferred stock repurchase amount	$ 25.0	$ 25.0
Dividend percentage of preferred stock		6.00%
Preferred stock dividend	$ 1.4
Minimum [Member]
Significant Accounting Policies [Line Items]
Payment terms					0 days
Minimum [Member] | Buildings [Member]
Significant Accounting Policies [Line Items]
Useful life of property, plant and equipment					20 years
Minimum [Member] | Machinery and Equipment [Member]
Significant Accounting Policies [Line Items]
Useful life of property, plant and equipment					10 years
Minimum [Member] | Other Fixed Assets [Member]
Significant Accounting Policies [Line Items]
Useful life of property, plant and equipment					3 years
Maximum [Member]
Significant Accounting Policies [Line Items]
Payment terms					90 days
Maximum [Member] | Buildings [Member]
Significant Accounting Policies [Line Items]
Useful life of property, plant and equipment					25 years
Maximum [Member] | Machinery and Equipment [Member]
Significant Accounting Policies [Line Items]
Useful life of property, plant and equipment					25 years
Maximum [Member] | Other Fixed Assets [Member]
Significant Accounting Policies [Line Items]
Useful life of property, plant and equipment					25 years