Debt (Details) - USD ($)			12 Months Ended
	Oct. 15, 2014	Feb. 18, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Debt Instrument [Line Items]
Debt Instrument Carrying Amount			$ 2,460,000,000	$ 2,460,000,000
Debt Instrument Unamortized Discount			4,612,000	4,014,000
Long-term Debt, Excluding Current Maturities			2,455,388,000	2,455,986,000
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity			1,300,000,000
Long-term Debt, Other Disclosures [Abstract]
Commercial Paper			$ 457,900,000	$ 196,700,000
Commercial Paper Weighted Average Interest Rate			0.42%	0.23%
Debt and Equity Securities Authorized for Issuance			$ 1,750,000,000
Debt And Equity Securities Available For Issuance			845,000,000
Stock Issued During Period, Shares, New Issues		9,200,000
Net proceeds from equity offering		$ 390,200,000	0	$ 390,205,000	$ 0
Long-term Debt, Fiscal Year Maturity [Abstract]
2016			0
2017			250,000,000
2018			0
2019			450,000,000
2020			0
Thereafter			1,760,000,000
Debt Instrument Carrying Amount			$ 2,460,000,000	$ 2,460,000,000
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			49.00%
Regulated Operation [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity			$ 1,300,000,000
Regulated Operation [Member] | Five Year Unsecured Revolving Credit Agreement [Member]
Line Of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity with Accordion Feature			1,500,000,000.0
Unregulated Operation [Member]
Line Of Credit Facility [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity			36,100,000
Minimum [Member] | Required by Covenant [Member]
Line Of Credit Facility [Line Items]
Excess of Debt			15,000,000
Maximum [Member] | Required by Covenant [Member]
Line Of Credit Facility [Line Items]
Excess of Debt			100,000,000
Unsecured Senior Notes Due 2014 [Member]
Debt Instrument [Line Items]
Debt Instrument Interest Rate Stated Percentage	4.95%			4.95%
Debt Instrument, Maturity Date				Oct. 15, 2014
Debt Instrument Carrying Amount	$ 500,000,000		0	$ 500,000,000
Extinguishment of Debt, Amount	500,000,000
Long-term Debt, Fiscal Year Maturity [Abstract]
Debt Instrument Carrying Amount	$ 500,000,000		$ 0	$ 500,000,000
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			$ 250,000,000	$ 250,000,000
Long-term Debt, Fiscal Year Maturity [Abstract]
Debt Instrument Carrying Amount			$ 250,000,000	$ 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
Long-term Debt, Fiscal Year Maturity [Abstract]
Debt Instrument Carrying Amount			$ 450,000,000	$ 450,000,000
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
Long-term Debt, Fiscal Year Maturity [Abstract]
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
Long-term Debt, Fiscal Year Maturity [Abstract]
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
Long-term Debt, Fiscal Year Maturity [Abstract]
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
Debt Instrument Carrying Amount	$ 500,000,000		$ 500,000,000	0
Debt Instrument Interest Rate Effective Percentage	4.086%
Net Proceeds From Issuance of Debt	$ 494,000,000
Long-term Debt, Fiscal Year Maturity [Abstract]
Debt Instrument Carrying Amount	$ 500,000,000		$ 500,000,000	$ 0
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
Long-term Debt, Fiscal Year Maturity [Abstract]
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
Long-term Debt, Fiscal Year Maturity [Abstract]
Debt Instrument Carrying Amount			150,000,000	$ 150,000,000
Commercial Paper [Member] | Regulated Operation [Member] | Five Year Unsecured Revolving Credit Agreement [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity			1,250,000,000
Uncommitted Revolving Credit Facility [Member] | Regulated Operation [Member] | Five Year Unsecured Revolving Credit Agreement [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity			1,250,000,000.00
Line Of Credit Facility Amount Outstanding			0
Line of Credit Facility, Remaining Borrowing Capacity			792,100,000
Line of Credit Facility, Accordion Feature			250,000,000
Uncommitted Revolving Credit Facility [Member] | Regulated Operation [Member] | Uncommitted Revolving Credit Facility and Promissory Note (AEH to AEC) [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity			500,000,000
Line Of Credit Facility Amount Outstanding			325,000,000
Uncommitted Revolving Credit Facility [Member] | Unregulated Operation [Member] | Uncommitted Revolving Credit Facility and Promissory Note (AEC to AEH) [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity			500,000,000
Line Of Credit Facility Amount Outstanding			$ 0
Debt Instrument, Basis Spread on Variable Rate			3.00%
Uncommitted Revolving Credit Facility [Member] | Unregulated Operation [Member] | $25 Million Uncommitted Facility [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity			$ 25,000,000
Uncommitted Revolving Credit Facility [Member] | Minimum [Member] | Regulated Operation [Member] | Five Year Unsecured Revolving Credit Agreement [Member]
Line Of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate			0.00%
Uncommitted Revolving Credit Facility [Member] | Maximum [Member] | Regulated Operation [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] | Regulated Operation [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
Debt Instrument, Basis Spread on Variable Rate			1.50%
Letters of Credit Outstanding, Amount			$ 5,900,000
Unsecured Revolving Credit Note [Member] | Unregulated Operation [Member] | $15 Million Revolving Credit Note [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity			15,000,000
Unsecured Loan Agreement [Member] | Regulated Operation [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