Debt - Narrative (Details) - USD ($)			3 Months Ended		6 Months Ended
	Mar. 04, 2019	Oct. 04, 2018	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018	Sep. 30, 2018
Line Of Credit Facility [Line Items]
Total long-term debt			$ 3,685,000,000		$ 3,685,000,000		$ 3,085,000,000
Maximum borrowing capacity			1,500,000,000		$ 1,500,000,000
Maximum debt-to-total-capitalization ratio					70.00%
Debt-to-total-capitalization ratio					41.00%
Net gain (loss) on settled interest rate agreements			(600,000)	$ (600,000)	$ 1,200,000	$ 1,200,000
Minimum
Line Of Credit Facility [Line Items]
Equity-to-total-capitalization ratio					50.00%
Outstanding indebtedness			15,000,000		$ 15,000,000
Maximum
Line Of Credit Facility [Line Items]
Equity-to-total-capitalization ratio					60.00%
Outstanding indebtedness			100,000,000		$ 100,000,000
Five Year Unsecured Revolving Credit Agreement
Line Of Credit Facility [Line Items]
Outstanding commercial paper			0		0		575,800,000
Five Year Unsecured Revolving Credit Agreement | Commercial Paper
Line Of Credit Facility [Line Items]
Maximum borrowing capacity			1,500,000,000.0		1,500,000,000.0
Accordion feature			250,000,000		250,000,000
Maximum borrowing capacity post accordion feature			1,750,000,000.00		1,750,000,000.00
$25 Million Bank Loan Agreement | Line of Credit
Line Of Credit Facility [Line Items]
Maximum borrowing capacity			25,000,000		25,000,000
Outstanding borrowings			0		0
$10 Million Revolving Credit Note | Revolving Credit Facility
Line Of Credit Facility [Line Items]
Maximum borrowing capacity			10,000,000		10,000,000
Remaining borrowing capacity			4,400,000		4,400,000
Unsecured 4.30% Senior Notes, due 2048
Line Of Credit Facility [Line Items]
Total long-term debt		$ 600,000,000	$ 600,000,000		$ 600,000,000		0
Interest rate		4.30%	4.30%		4.30%
Proceeds from Issuance of Debt		$ 590,600,000
Effective interest rate		4.37%
Unsecured 4.125% Senior Notes, due 2049
Line Of Credit Facility [Line Items]
Total long-term debt	$ 450,000,000		$ 450,000,000		$ 450,000,000		0
Interest rate	4.125%		4.125%		4.125%
Proceeds from Issuance of Debt	$ 443,400,000
Effective interest rate	4.86%
Unsecured 8.50% Senior Notes, due March 2019
Line Of Credit Facility [Line Items]
Total long-term debt			$ 0		$ 0		$ 450,000,000
Interest rate							8.50%
Interest Rate Swap
Line Of Credit Facility [Line Items]
Net gain (loss) on settled interest rate agreements	$ 90,100,000
LIBOR | Five Year Unsecured Revolving Credit Agreement | Commercial Paper | Minimum
Line Of Credit Facility [Line Items]
Interest rate spread					0.00%
LIBOR | Five Year Unsecured Revolving Credit Agreement | Commercial Paper | Maximum
Line Of Credit Facility [Line Items]
Interest rate spread					1.25%