Debt (Details) (USD $)	0 Months Ended	3 Months Ended	12 Months Ended
	Oct. 15, 2014	Mar. 31, 2015	Sep. 30, 2014
Debt Instrument [Line Items]
Net Proceeds From Issuance Of Debt	$ 494,000,000
Debt Instrument Carrying Amount		2,460,000,000	2,460,000,000
Debt Instrument, Unamortized Discount		4,783,000	4,014,000
Long-term debt		2,455,217,000	2,455,986,000
Extinguishment of Debt, Amount	500,000,000
Extinguishment of Debt, Interest Terms	4.95%
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity		1,300,000,000
Debt Instrument, Basis Spread on Variable Rate		3.00%
Long Term Debt Other Disclosures [Abstract]
Commercial Paper		225,000,000	196,700,000
Debt and Equity Securities Authorized for Issuance		1,750,000,000.00
Debt And Equity Securities Available For Issuance		845,000,000
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%
Regulated Operation [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity		1,300,000,000
Unregulated Operation [Member]
Line Of Credit Facility [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity		34,900,000
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
Five Year Unsecured Revolving Credit Agreement [Member] | Regulated Operation [Member]
Line Of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity with Accordion Feature		1,500,000,000
Five Year Unsecured Revolving Credit Agreement [Member] | Commercial Paper [Member] | Regulated Operation [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity		1,250,000,000
Five Year Unsecured Revolving Credit Agreement [Member] | Uncommitted Revolving Credit Facility [Member] | Regulated Operation [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity		1,250,000,000
$15 Million Revolving Credit Note [Member] | Unsecured Revolving Credit Note [Member] | Unregulated Operation [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity		15,000,000
$25 Million Bank Loan Agreement [Member] | Unsecured Loan Agreement [Member] | Regulated Operation [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity		25,000,000
$10 Million Revolving Credit Note [Member] | Unsecured Revolving Credit Note [Member] | Regulated Operation [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity		10,000,000
Line of Credit Facility, Remaining Borrowing Capacity		4,100,000
Uncommitted Revolving Credit Facility and Promissory Note (AEH to AEC) [Member] | Uncommitted Revolving Credit Facility [Member] | Regulated Operation [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity		500,000,000
Uncommitted Revolving Credit Facility and Promissory Note (AEC to AEH) [Member] | Uncommitted Revolving Credit Facility [Member] | Unregulated Operation [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity		500,000,000
Debt Instrument, Basis Spread on Variable Rate		3.00%
$25 Million Uncommitted Facility [Member] | Uncommitted Revolving Credit Facility [Member] | Unregulated Operation [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity		25,000,000
Unsecured Senior Notes Due 2014 [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount		0	500,000,000
Debt Instrument, Interest Rate, Stated Percentage		4.95%	4.95%
Debt Instrument, Maturity Date		Oct. 15, 2014	Oct. 15, 2014
Unsecured Senior Notes Due 2017 [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount		250,000,000	250,000,000
Debt Instrument, Interest Rate, Stated Percentage		6.35%	6.35%
Debt Instrument, Maturity Date		Jun. 15, 2017	Jun. 15, 2017
Unsecured Senior Notes Due 2019 [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount		450,000,000	450,000,000
Debt Instrument, Interest Rate, Stated Percentage		8.50%	8.50%
Debt Instrument, Maturity Date		Mar. 15, 2019	Mar. 15, 2019
Unsecured Senior Notes Due 2034 [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount		200,000,000	200,000,000
Debt Instrument, Interest Rate, Stated Percentage		5.95%	5.95%
Debt Instrument, Maturity Date		Oct. 15, 2034	Oct. 15, 2034
Unsecured Senior Notes Due 2041 [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount		400,000,000	400,000,000
Debt Instrument, Interest Rate, Stated Percentage		5.50%	5.50%
Debt Instrument, Maturity Date		Jun. 15, 2041	Jun. 15, 2041
Unsecured Senior Notes Due 2043 [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount		500,000,000	500,000,000
Debt Instrument, Interest Rate, Stated Percentage		4.15%	4.15%
Debt Instrument, Maturity Date		Jan. 15, 2043	Jan. 15, 2043
Unsecured Senior Notes Due 2044 [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount	500,000,000	500,000,000	0
Debt Instrument, Interest Rate, Stated Percentage	4.125%	4.125%	0.00%
Debt Instrument, Maturity Date		Oct. 15, 2044	Oct. 14, 2044
Debt Instrument, Interest Rate, Effective Percentage	4.086%
Medium Term Notes Due 2025 [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount		10,000,000	10,000,000
Debt Instrument, Interest Rate, Stated Percentage		6.67%	6.67%
Debt Instrument, Maturity Date		Dec. 15, 2025	Dec. 15, 2025
Unsecured Debentures Due 2028 [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount		$ 150,000,000	$ 150,000,000
Debt Instrument, Interest Rate, Stated Percentage		6.75%	6.75%
Debt Instrument, Maturity Date		Jul. 15, 2028	Jul. 15, 2028