Subsidiary Debt - Northern Powergrid (Details) - USD ($) $ in Millions		Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Par value		$ 36,295
Subsidiary Debt [Member]
Debt Instrument [Line Items]
Par value		27,476
Other long-term debt		27,354	$ 27,214
Northern Powergrid Holdings [Member] | Subsidiary Debt [Member]
Debt Instrument [Line Items]
Par value	[1]	2,351
Other long-term debt		2,379	2,772
Northern Powergrid Holdings [Member] | Subsidiary Debt [Member] | Bonds, 8.875%, due 2020 [Member]
Debt Instrument [Line Items]
Par value	[1]	123
Other long-term debt		$ 136	$ 162
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]	$ 247
Other long-term debt		$ 259	$ 315
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]	$ 333
Other long-term debt		$ 333	$ 398
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]	$ 247
Other long-term debt		$ 257	$ 306
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]	$ 185
Other long-term debt		$ 182	$ 217
Debt Instrument, Interest Rate, Stated Percentage		2.50%	2.50%
Northern Powergrid Holdings [Member] | Subsidiary Debt [Member] | European Investment Bank loan, 2.073%, due 2025 [Member]
Debt Instrument [Line Items]
Par value	[1]	$ 62
Other long-term debt		62	$ 0
Northern Powergrid Holdings [Member] | Subsidiary Debt [Member] | European Investment Bank loans, 2.564%, due 2017 [Member]
Debt Instrument [Line Items]
Par value	[1]	308
Other long-term debt		$ 308	$ 368
Debt Instrument, Interest Rate, Stated Percentage		2.564%	2.564%
Northern Powergrid Holdings [Member] | Subsidiary Debt [Member] | Bonds, 7.25%, due 2028 [Member]
Debt Instrument [Line Items]
Par value	[1]	$ 229
Other long-term debt		$ 234	$ 280
Debt Instrument, Interest Rate, Stated Percentage		7.25%	7.25%
Northern Powergrid Holdings [Member] | Subsidiary Debt [Member] | Bonds, 5.125%, due 2035 [Member]
Debt Instrument [Line Items]
Par value	[1]	$ 247
Other long-term debt		$ 243	$ 291
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]	$ 185
Other long-term debt		$ 183	$ 218
Debt Instrument, Interest Rate, Stated Percentage		5.125%	5.125%
Northern Powergrid Holdings [Member] | Subsidiary Debt [Member] | Bonds, 4.375%, due 2032 [Member]
Debt Instrument [Line Items]
Par value	[1]	$ 185
Other long-term debt		$ 182	$ 217
Debt Instrument, Interest Rate, Stated Percentage		4.375%	4.375%

[1] The par values for these debt instruments are denominated in sterling.