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Subsidiary Debt - Northern Powergrid (Details) - USD ($)
$ in Millions
Dec. 31, 2015
Dec. 31, 2014
Debt Instrument [Line Items]    
Par value $ 38,133  
Subsidiary Debt [Member]    
Debt Instrument [Line Items]    
Par value 27,314  
Other long-term debt 27,214 $ 26,848
Northern Powergrid Holdings [Member] | Subsidiary Debt [Member]    
Debt Instrument [Line Items]    
Par value [1] 2,735  
Other long-term debt 2,772 2,317
Northern Powergrid Holdings [Member] | Subsidiary Debt [Member] | Bonds, 8.875%, due 2020 [Member]    
Debt Instrument [Line Items]    
Par value [1] 147  
Other long-term debt $ 162 $ 172
Debt Instrument, Interest Rate, Stated Percentage 8.875% 8.875%
Northern Powergrid Holdings [Member] | Subsidiary Debt [Member] | Bonds, 9.25%, due 2020 [Member]    
Debt Instrument [Line Items]    
Par value [1] $ 295  
Other long-term debt $ 315 $ 338
Debt Instrument, Interest Rate, Stated Percentage 9.25% 9.25%
Northern Powergrid Holdings [Member] | Subsidiary Debt [Member] | European Investment Bank loans, 3.901% to 4.586%, due 2018 to 2022 [Member]    
Debt Instrument [Line Items]    
Par value [1] $ 398  
Other long-term debt $ 398 $ 420
Northern Powergrid Holdings [Member] | Subsidiary Debt [Member] | European Investment Bank loans, 3.901% to 4.586%, due 2018 to 2022 [Member] | Minimum [Member]    
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Stated Percentage 3.901% 3.901%
Northern Powergrid Holdings [Member] | Subsidiary Debt [Member] | European Investment Bank loans, 3.901% to 4.586%, due 2018 to 2022 [Member] | Maximum [Member]    
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Stated Percentage 4.586% 4.586%
Northern Powergrid Holdings [Member] | Subsidiary Debt [Member] | Bonds, 7.25%, due 2022 [Member]    
Debt Instrument [Line Items]    
Par value [1] $ 295  
Other long-term debt $ 306 $ 324
Debt Instrument, Interest Rate, Stated Percentage 7.25% 7.25%
Northern Powergrid Holdings [Member] | Subsidiary Debt [Member] | Bonds, 2.50%, due 2025 [Member]    
Debt Instrument [Line Items]    
Par value [1] $ 221  
Other long-term debt $ 217 $ 0
Debt Instrument, Interest Rate, Stated Percentage 2.50% 0.00%
Northern Powergrid Holdings [Member] | Subsidiary Debt [Member] | European Investment Bank loans, 2.564%, due 2017 [Member]    
Debt Instrument [Line Items]    
Par value [1] $ 369  
Other long-term debt $ 368 $ 0
Debt Instrument, Interest Rate, Stated Percentage 2.564% 0.00%
Northern Powergrid Holdings [Member] | Subsidiary Debt [Member] | Bonds, 7.25%, due 2028 [Member]    
Debt Instrument [Line Items]    
Par value [1] $ 273  
Other long-term debt $ 280 $ 297
Debt Instrument, Interest Rate, Stated Percentage 7.25% 7.25%
Northern Powergrid Holdings [Member] | Subsidiary Debt [Member] | Bonds, 4.375%, due 2032 [Member]    
Debt Instrument [Line Items]    
Par value [1] $ 221  
Other long-term debt $ 217 $ 229
Debt Instrument, Interest Rate, Stated Percentage 4.375% 4.375%
Northern Powergrid Holdings [Member] | Subsidiary Debt [Member] | Bonds, 5.125%, due 2035 [Member]    
Debt Instrument [Line Items]    
Par value [1] $ 295  
Other long-term debt $ 291 $ 307
Debt Instrument, Interest Rate, Stated Percentage 5.125% 5.125%
Northern Powergrid Holdings [Member] | Subsidiary Debt [Member] | Bonds, 5.125%, due 2035 [Member]    
Debt Instrument [Line Items]    
Par value [1] $ 221  
Other long-term debt $ 218 $ 230
Debt Instrument, Interest Rate, Stated Percentage 5.125% 5.125%
[1] The par values for these debt instruments are denominated in sterling.