Subsidiary Debt - Northern Powergrid (Details) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Par value		$ 35,376
Subsidiary Debt [Member]
Debt Instrument [Line Items]
Par value		28,780
Other long-term debt		28,641	$ 27,354
Northern Powergrid Holdings [Member] | Subsidiary Debt [Member]
Debt Instrument [Line Items]
Par value	[1]	2,792
Other long-term debt		2,805	2,379
Northern Powergrid Holdings [Member] | Subsidiary Debt [Member] | Bonds, 8.875%, due 2020 [Member]
Debt Instrument [Line Items]
Par value	[1]	135
Other long-term debt		$ 144	$ 136
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]	$ 270
Other long-term debt		$ 279	$ 259
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]	$ 366
Other long-term debt		$ 366	$ 333
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]	$ 270
Other long-term debt		$ 279	$ 257
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]	$ 203
Other long-term debt		$ 200	$ 182
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]	$ 68
Other long-term debt		$ 69	$ 62
Debt Instrument, Interest Rate, Stated Percentage		2.073%	2.073%
Northern Powergrid Holdings [Member] | Subsidiary Debt [Member] | European Investment Bank loans, 2.564%, due 2017 [Member]
Debt Instrument [Line Items]
Par value	[1]	$ 338
Other long-term debt		$ 336	$ 308
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]	$ 250
Other long-term debt		$ 256	$ 234
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]	$ 270
Other long-term debt		$ 267	$ 243
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]	$ 203
Other long-term debt		$ 200	$ 183
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]	$ 203
Other long-term debt		$ 199	$ 182
Debt Instrument, Interest Rate, Stated Percentage		4.375%	4.375%
Northern Powergrid Holdings [Member] | Subsidiary Debt [Member] | Variable-rate bond, due 2026 [Member]
Debt Instrument [Line Items]
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate		2.27%
Par value	[1],[2]	$ 216
Other long-term debt	[2]	$ 210	$ 0
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate		2.82%

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

[2] (2)Amortizes semiannually and the Company has entered into an interest rate swap that fixes the interest rate on 85% of the outstanding debt. The variable interest rate as of December 31, 2017 was 2.27% while the fixed interest rate was 2.82%.