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Debt (Details) - USD ($)
3 Months Ended 12 Months Ended
Oct. 15, 2014
Jun. 30, 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   4,697,000 4,014,000
Long-term debt   2,455,303,000 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   $ 252,000,000 196,700,000
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   47.00%  
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   36,000,000  
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   1,500,000,000.0  
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   4,200,000  
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. 15, 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