Risk Management and Hedging Activities - Derivative Contract Volumes (Details) £ in Millions, gal in Millions, MWh in Millions, Dth in Millions, $ in Millions	Sep. 30, 2017 USD ($) MWh gal Dth	Sep. 30, 2017 GBP (£) MWh gal Dth	Dec. 31, 2016 USD ($) MWh gal Dth	Dec. 31, 2016 GBP (£) MWh gal Dth
Electricity purchases (in megawatt hours) [Member] | Commodity derivative [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, nonmonetary notional amount | MWh	9	9	5	5
Natural gas purchases (in decatherms) [Member] | Commodity derivative [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, nonmonetary notional amount | Dth	339	339	271	271
Fuel purchases (in gallons) [Member] | Commodity derivative [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, nonmonetary notional amount | gal	2	2	11	11
Mortgage sale commitments, net [Member] | Interest Rate Contract [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, notional amount	$ (442)		$ (309)
United States of America, Dollars | Interest rate swap [Member] | Interest Rate Contract [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, notional amount	$ (694)		$ (714)
United Kingdom, Pounds | Interest rate swap [Member] | Interest Rate Contract [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, notional amount | £		£ (102)		£ 0