Debt (Details) - USD ($)	12 Months Ended
Sep. 30, 2017	Sep. 30, 2016
Debt Instrument [Line Items]
Debt Instrument Carrying Amount	$ 3,085,000,000	$ 2,460,000,000
Debt Instrument, Unamortized Discount (Premium), Net	(4,384,000)	4,270,000
Debt Issuance Costs, Net	22,339,000	16,951,000
Current maturities of long-term debt	0	250,000,000
Long-term Debt, Excluding Current Maturities	3,067,045,000	2,188,779,000
Net Proceeds From Issuance of Debt	753,000,000
Loss on settlement of interest rate swaps	37,000,000
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity	1,500,000,000
Long-term Debt, Other Disclosures [Abstract]
Commercial Paper	$ 447,700,000	$ 829,800,000
Commercial Paper Weighted Average Interest Rate	1.25%	0.81%
Term Loan Agreement, Maximum Borrowing Capacity	$ 200,000,000
Term Loan Agreement, Incremental Borrowings	1,000,000
Term Loan Agreement, Amount Outstanding	125,000,000	$ 0
Long-term Debt, Fiscal Year Maturity [Abstract]
2018	0
2019	575,000,000
2020	0
2021	0
2022	0
Thereafter	$ 2,510,000,000
Required by Covenant [Member]
Line Of Credit Facility [Line Items]
Ratio of Total Debt to Total Capital	70.00%
Actual [Member]
Line Of Credit Facility [Line Items]
Ratio of Total Debt to Total Capital	48.00%
Minimum [Member]
Line Of Credit Facility [Line Items]
Equity to Capitalization Ratio	50.00%
Minimum [Member] \| Required by Covenant [Member]
Line Of Credit Facility [Line Items]
Excess of Debt	$ 15,000,000
Maximum [Member]
Line Of Credit Facility [Line Items]
Equity to Capitalization Ratio	60.00%
Maximum [Member] \| Required by Covenant [Member]
Line Of Credit Facility [Line Items]
Excess of Debt	$ 100,000,000
Unsecured Senior Notes Due 2017 [Member]
Debt Instrument [Line Items]
Debt Instrument Interest Rate Stated Percentage	6.35%	6.35%
Debt Instrument, Maturity Date	Jun. 15, 2017	Jun. 15, 2017
Debt Instrument Carrying Amount	$ 0	$ 250,000,000
Unsecured Senior Notes Due 2019 [Member]
Debt Instrument [Line Items]
Debt Instrument Interest Rate Stated Percentage	8.50%	8.50%
Debt Instrument, Maturity Date	Mar. 15, 2019	Mar. 15, 2019
Debt Instrument Carrying Amount	$ 450,000,000	$ 450,000,000
Unsecured Senior Notes Due 2027 [Member]
Debt Instrument [Line Items]
Debt Instrument Interest Rate Stated Percentage	3.00%	3.00%
Debt Instrument, Maturity Date	Jun. 15, 2027	Jun. 15, 2027
Debt Instrument Carrying Amount	$ 500,000,000	$ 0
Debt Instrument Interest Rate Effective Percentage	3.12%
Unsecured Senior Notes Due 2034 [Member]
Debt Instrument [Line Items]
Debt Instrument Interest Rate Stated Percentage	5.95%	5.95%
Debt Instrument, Maturity Date	Oct. 15, 2034	Oct. 15, 2034
Debt Instrument Carrying Amount	$ 200,000,000	$ 200,000,000
Unsecured Senior Notes Due 2041 [Member]
Debt Instrument [Line Items]
Debt Instrument Interest Rate Stated Percentage	5.50%	5.50%
Debt Instrument, Maturity Date	Jun. 15, 2041	Jun. 15, 2041
Debt Instrument Carrying Amount	$ 400,000,000	$ 400,000,000
Unsecured Senior Notes Due 2043 [Member]
Debt Instrument [Line Items]
Debt Instrument Interest Rate Stated Percentage	4.15%	4.15%
Debt Instrument, Maturity Date	Jan. 15, 2043	Jan. 15, 2043
Debt Instrument Carrying Amount	$ 500,000,000	$ 500,000,000
Unsecured Senior Notes Due 2044 [Member]
Debt Instrument [Line Items]
Debt Instrument Interest Rate Stated Percentage	4.125%	4.125%
Debt Instrument, Maturity Date	Oct. 15, 2044	Oct. 15, 2044
Debt Instrument Carrying Amount	$ 750,000,000	$ 500,000,000
Debt Instrument Interest Rate Effective Percentage	4.40%
Debt Instrument, Increase (Decrease), Net	$ 250,000,000
Medium Term Notes Due 2025 [Member]
Debt Instrument [Line Items]
Debt Instrument Interest Rate Stated Percentage	6.67%	6.67%
Debt Instrument, Maturity Date	Dec. 15, 2025	Dec. 15, 2025
Debt Instrument Carrying Amount	$ 10,000,000	$ 10,000,000
Unsecured Debentures Due 2028 [Member]
Debt Instrument [Line Items]
Debt Instrument Interest Rate Stated Percentage	6.75%	6.75%
Debt Instrument, Maturity Date	Jul. 15, 2028	Jul. 15, 2028
Debt Instrument Carrying Amount	$ 150,000,000	$ 150,000,000
Multi-Draw Term Loan [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount	125,000,000	0
Commercial Paper [Member] \| Five Year Unsecured Revolving Credit Agreement [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity	1,500,000,000	$ 1,250,000,000.00
Line Of Credit Facility Amount Outstanding	1,500,000,000.0
Line of Credit Facility, Accordion Feature	250,000,000
Line of Credit Facility, Maximum Borrowing Capacity with Accordion Feature	$ 1,750,000,000.00
Commercial Paper [Member] \| Minimum [Member] \| Five Year Unsecured Revolving Credit Agreement [Member]
Line Of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.00%
Commercial Paper [Member] \| Maximum [Member] \| Five Year Unsecured Revolving Credit Agreement [Member]
Line Of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.25%
Unsecured Revolving Credit Note [Member] \| $10 Million Revolving Credit Note [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity	$ 10,000,000
Line Of Credit Facility Amount Outstanding	0
Line of Credit Facility, Remaining Borrowing Capacity	4,200,000
Unsecured Loan Agreement [Member] \| $25 Million Bank Loan Agreement [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity	25,000,000
Line Of Credit Facility Amount Outstanding	$ 0

Weighted average interest rate of all outstanding commercial paper.

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Equity to Capitalization Ratio

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Total outstanding indebtedness arising out of any other credit agreements, as stated in the public debt indentures relating to senior notes and debentures, as well as certain revolving credit agreements.

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Maximum available borrowing capacity under the credit facility with consideration of any additional commited line portions under the accordion feature that could be borrowed against with the approval of the issuing banks.

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Monetary value of the accordion feature, the additional line of credit portion that could be borrowed against with the approval of the issuing banks.

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The net proceeds from the issuance of the debt.

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For covenant purposes, total debt divided by total capitalization including original issue discount, short-term debt, capital leases, operating leases and guaranty obligations.

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The amount outstanding for the multi-draw term loan agreement.

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Carrying value as of the balance sheet date of short-term borrowings using unsecured obligations issued by banks, corporations and other borrowers to investors. The maturities of these money market securities generally do not exceed 270 days.

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Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Net increase or decrease in the carrying amount of the debt instrument for the period.

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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

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Amount, after accumulated amortization, of debt discount (premium).

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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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