Major Accounting Policies (Narrative) (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Sep. 30, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Accounting Policies [Abstract]
Assets of discontinued operations			$ 0	$ 0
Liabilities of discontinued operations			0	0
Environmental expenditures			18,300,000	14,200,000	$ 12,800,000
Pre-tax loss from discontinued operations	$ 19,000,000.0		19,000,000.0
Capitalized interest		$ 19,500,000	15,900,000	13,500,000	19,500,000
Asset retirement obligations		190,400,000	241,400,000	208,200,000	190,400,000
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease in cost of sales			(5,858,100,000)	$ (5,975,500,000)	(6,189,500,000)
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, residual value			$ 0
Computer software costs | Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life			3 years
Computer software costs | Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life			5 years
Customer relationships | Minimum
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, useful life			5 years
Customer relationships | Maximum
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, useful life			25 years
Patents and technology | Minimum
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, useful life			5 years
Patents and technology | Maximum
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, useful life			15 years
Land Use Rights
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, useful life			50 years
Change in Inventory Valuation Method
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease in cost of sales		$ 24,100,000			$ 24,100,000