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Debt (Details) - USD ($)
3 Months Ended
Mar. 31, 2018
Sep. 30, 2017
Debt Instrument [Line Items]    
Debt Instrument Carrying Amount $ 3,085,000,000 $ 3,085,000,000
Debt Instrument, Unamortized Discount (Premium), Net (4,412,000) (4,384,000)
Debt Issuance Costs, Net 21,520,000 22,339,000
Current maturities of long-term debt 450,000,000 0
Long-term debt 2,617,892,000 3,067,045,000
Line Of Credit Facility [Line Items]    
Line of Credit Facility, Maximum Borrowing Capacity 1,500,000,000  
Long Term Debt Other Disclosures [Abstract]    
Term Loan Agreement, Maximum Borrowing Capacity 200,000,000 200,000,000
Commercial Paper $ 129,600,000 447,700,000
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 41.00%  
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  
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 1,500,000,000.0  
Line of Credit Facility, Accordion Feature 250,000,000  
Line of Credit Facility, Maximum Borrowing Capacity with Accordion Feature $ 1,750,000,000.00  
Five Year Unsecured Revolving Credit Agreement [Member] | Commercial Paper [Member] | Minimum [Member] | Regulated Operation [Member]    
Line Of Credit Facility [Line Items]    
Debt Instrument, Basis Spread on Variable Rate 0.00%  
Five Year Unsecured Revolving Credit Agreement [Member] | Commercial Paper [Member] | Maximum [Member] | Regulated Operation [Member]    
Line Of Credit Facility [Line Items]    
Debt Instrument, Basis Spread on Variable Rate 1.25%  
$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  
Line of Credit Facility, Fair Value of Amount Outstanding 0  
$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 4,400,000  
Line of Credit Facility, Fair Value of Amount Outstanding 0  
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 2027 [Member]    
Debt Instrument [Line Items]    
Debt Instrument Carrying Amount $ 500,000,000 $ 500,000,000
Debt Instrument, Interest Rate, Stated Percentage 3.00% 3.00%
Debt Instrument, Maturity Date Jun. 15, 2027 Jun. 15, 2027
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 $ 750,000,000 $ 750,000,000
Debt Instrument, Interest Rate, Stated Percentage 4.125% 4.125%
Debt Instrument, Maturity Date Oct. 15, 2044 Oct. 15, 2044
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
Multi-Draw Term Loan Due 2019 [Member]    
Debt Instrument [Line Items]    
Debt Instrument Carrying Amount [1] $ 125,000,000 $ 125,000,000
Debt Instrument, Maturity Date Sep. 22, 2019 Sep. 22, 2019
[1] Up to $200 million can be drawn under this term loan.