Risk Management and Hedging Activities - Derivative Contract Volumes (Details) gal in Millions, MWh in Millions, Dth in Millions, $ in Millions	Dec. 31, 2015 USD ($) gal Dth MWh	Dec. 31, 2014 USD ($) 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	10	6
Commodity Contract [Member] | Natural gas purchases, in decatherms [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, Nonmonetary Notional Amount | Dth	317	308
Commodity Contract [Member] | Fuel purchases (in gallons) [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, Nonmonetary Notional Amount | gal	11	2
Interest Rate Contract [Member] | Interest Rate Swap [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, Notional Amount	$ (653)	$ (443)
Interest Rate Contract [Member] | Mortgage Sale Commitments, Net [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, Notional Amount	$ (312)	$ (264)