Debt - Schedule of Long-term Debt Instruments (Details) - USD ($)	Jun. 30, 2018	Sep. 30, 2017
Debt Instrument [Line Items]
Total long-term debt	$ 3,085,000,000	$ 3,085,000,000
Original issue (premium) / discount on unsecured senior notes and debentures	(4,425,000)	(4,384,000)
Debt issuance cost	21,110,000	22,339,000
Current maturities	450,000,000	0
Long-term debt	2,618,315,000	3,067,045,000
Unsecured 8.50% Senior Notes, due March 2019
Debt Instrument [Line Items]
Total long-term debt	$ 450,000,000	$ 450,000,000
Interest rate	8.50%	8.50%
Unsecured 3.00% Senior Notes, due 2027
Debt Instrument [Line Items]
Total long-term debt	$ 500,000,000	$ 500,000,000
Interest rate	3.00%	3.00%
Unsecured 5.95% Senior Notes, due 2034
Debt Instrument [Line Items]
Total long-term debt	$ 200,000,000	$ 200,000,000
Interest rate	5.95%	5.95%
Unsecured 5.50% Senior Notes, due 2041
Debt Instrument [Line Items]
Total long-term debt	$ 400,000,000	$ 400,000,000
Interest rate	5.50%	5.50%
Unsecured 4.15% Senior Notes, due 2043
Debt Instrument [Line Items]
Total long-term debt	$ 500,000,000	$ 500,000,000
Interest rate	4.15%	4.15%
Unsecured 4.125% Senior Notes, due 2044
Debt Instrument [Line Items]
Total long-term debt	$ 750,000,000	$ 750,000,000
Interest rate	4.125%	4.125%
Medium-term note Series A, 1995-1, 6.67%, due 2025
Debt Instrument [Line Items]
Total long-term debt	$ 10,000,000	$ 10,000,000
Interest rate	6.67%	6.67%
Unsecured 6.75% Debentures, due 2028
Debt Instrument [Line Items]
Total long-term debt	$ 150,000,000	$ 150,000,000
Interest rate	6.75%	6.75%
Floating-rate term loan, due September 2019
Debt Instrument [Line Items]
Total long-term debt	$ 125,000,000	$ 125,000,000
Maximum borrowing capacity	$ 200,000,000	$ 200,000,000