Risk Management and Hedging Activities - Derivative Contract Volumes (Details) gal in Millions, MWh in Millions, Dth in Millions, $ in Millions	Dec. 31, 2017 USD ($) gal Dth MWh	Dec. 31, 2016 USD ($) gal Dth MWh
Commodity Contract [Member] | Electricity purchases (sales), net, in megawatt hours [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, Nonmonetary Notional Amount | MWh	4	5
Commodity Contract [Member] | Natural gas purchases, in decatherms [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, Nonmonetary Notional Amount | Dth	310	271
Commodity Contract [Member] | Fuel purchases (in gallons) [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, Nonmonetary Notional Amount | gal	0	11
Interest Rate Contract [Member] | Mortgage Sale Commitments, Net [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, Notional Amount	$ (422)	$ (309)
United States of America, Dollars | Interest Rate Contract [Member] | Interest Rate Swap [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, Notional Amount	(679)	(714)
United Kingdom, Pounds | Interest Rate Contract [Member] | Interest Rate Swap [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, Notional Amount	$ (136)	$ 0