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Debt (Details) - USD ($)
3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Sep. 30, 2017
Debt Instrument [Line Items]      
Debt Instrument Carrying Amount $ 3,085,000,000 $ 3,085,000,000 $ 3,085,000,000
Debt Instrument, Unamortized Discount (Premium), Net (4,398,000) (4,384,000) (4,384,000)
Debt Issuance Costs, Net 21,929,000 22,339,000 22,339,000
Long-term debt 3,067,469,000 3,067,045,000 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    
Commercial Paper $ 336,800,000 447,700,000 447,700,000
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 44.00%    
Minimum [Member]      
Line Of Credit Facility [Line Items]      
Ratio of Total Equity to Total Capital 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]      
Ratio of Total Equity to Total Capital 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    
Line of Credit Facility, Expiration Date Sep. 25, 2021    
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 $ 450,000,000
Debt Instrument, Interest Rate, Stated Percentage 8.50% 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 $ 500,000,000
Debt Instrument, Interest Rate, Stated Percentage 3.00% 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 $ 200,000,000
Debt Instrument, Interest Rate, Stated Percentage 5.95% 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 $ 400,000,000
Debt Instrument, Interest Rate, Stated Percentage 5.50% 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 $ 500,000,000
Debt Instrument, Interest Rate, Stated Percentage 4.15% 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 $ 750,000,000
Debt Instrument, Interest Rate, Stated Percentage 4.125% 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 $ 10,000,000
Debt Instrument, Interest Rate, Stated Percentage 6.67% 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 $ 150,000,000
Debt Instrument, Interest Rate, Stated Percentage 6.75% 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 $ 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.