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Financing Activities - Long-term Debt and ATM Program (Details)
$ / shares in Units, shares in Thousands, £ in Millions, $ in Millions
1 Months Ended 3 Months Ended
Apr. 30, 2017
USD ($)
Mar. 31, 2017
USD ($)
Mar. 31, 2017
GBP (£)
Feb. 26, 2015
USD ($)
Integer
Mar. 31, 2017
USD ($)
$ / shares
shares
Mar. 31, 2016
USD ($)
$ / shares
shares
At-the-Market Stock Offering Program (Numeric) [Abstract]            
Net proceeds from issuance of common stock         $ 73 $ 42
WPD South Wales PLC [Member] | Index Linked Senior Unsecured Notes [Member]            
Long-term Debt (Numeric) [Abstract]            
Principal amount | £     £ 50      
Stated interest rate     0.01%      
Proceeds from debt, net of issuance costs | £     £ 53      
Maturity date   Mar. 14, 2029 Mar. 14, 2029      
Proceeds from debt, including premium net of debt issuance costs   $ 64        
Louisville Gas And Electric Company [Member] | Senior Debt Obligations [Member] | Pollution Control Revenue Bonds, 2003 Series A [Member] | Remarketed [Member] | Subsequent Event [Member]            
Long-term Debt (Numeric) [Abstract]            
Principal amount $ 128          
Stated interest rate 1.50%          
Maturity date Oct. 01, 2033          
Repurchase date of debt instruments Apr. 01, 2019          
LG And E And KU Energy LLC [Member] | Louisville Gas And Electric Company [Member] | Senior Debt Obligations [Member] | Pollution Control Revenue Bonds, 2003 Series A [Member] | Remarketed [Member] | Subsequent Event [Member]            
Long-term Debt (Numeric) [Abstract]            
Principal amount $ 128          
Stated interest rate 1.50%          
Maturity date Oct. 01, 2033          
Repurchase date of debt instruments Apr. 01, 2019          
Louisville Gas And Electric Co [Member] | Senior Debt Obligations [Member] | Pollution Control Revenue Bonds, 2003 Series A [Member] | Remarketed [Member] | Subsequent Event [Member]            
Long-term Debt (Numeric) [Abstract]            
Principal amount $ 128          
Stated interest rate 1.50%          
Maturity date Oct. 01, 2033          
Repurchase date of debt instruments Apr. 01, 2019          
At The Market Stock Offering Program [Member]            
Equity Programs [Line Items]            
Stock Issued During Period, Shares, New Issues | shares         1,364 0
At-the-Market Stock Offering Program (Numeric) [Abstract]            
Number of distribution agreements | Integer       2    
Aggregate sales price of common stock based on two separate equity distribution agreements       $ 500    
Average share price | $ / shares         $ 36.66 $ 0.00
Net proceeds from issuance of common stock         $ 50 $ 0