Debt - Narrative (Details) - USD ($)	3 Months Ended
	Dec. 31, 2018	Sep. 30, 2018
Line Of Credit Facility [Line Items]
Total long-term debt	$ 3,685,000,000	$ 3,085,000,000
Maximum borrowing capacity	$ 1,500,000,000
Maximum debt-to-total-capitalization ratio	70.00%
Debt-to-total-capitalization ratio	42.00%
Minimum
Line Of Credit Facility [Line Items]
Equity-to-total-capitalization ratio	50.00%
Outstanding indebtedness	$ 15,000,000
Maximum
Line Of Credit Facility [Line Items]
Equity-to-total-capitalization ratio	60.00%
Outstanding indebtedness	$ 100,000,000
Five Year Unsecured Revolving Credit Agreement
Line Of Credit Facility [Line Items]
Outstanding commercial paper	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
Accordion feature	250,000,000
Maximum borrowing capacity post accordion feature	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
Outstanding borrowings	0
$10 Million Revolving Credit Note | Revolving Credit Facility
Line Of Credit Facility [Line Items]
Maximum borrowing capacity	10,000,000
Remaining borrowing capacity	4,400,000
Unsecured 4.30% Senior Notes, due 2048 [Member]
Line Of Credit Facility [Line Items]
Total long-term debt	$ 600,000,000	$ 0
Interest rate	4.30%	4.30%
Proceeds from Issuance of Debt	$ 590,600,000
Effective interest rate	4.37%
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%