Risk Management and Hedging Activities - Derivative Contract Volumes (Details) gal in Millions, MWh in Millions, Dth in Millions, $ in Millions	Jun. 30, 2016 USD ($) MWh gal Dth	Dec. 31, 2015 USD ($) 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	7	10
Natural gas purchases (in decatherms) [Member] | Commodity derivative [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, nonmonetary notional amount | Dth	311	317
Fuel purchases (in gallons) [Member] | Commodity derivative [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, nonmonetary notional amount | gal	6	11
Interest rate swap [Member] | Interest Rate Contract [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, notional amount	$ (730)	$ (653)
Mortgage sale commitments, net [Member] | Interest Rate Contract [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, notional amount	$ (464)	$ (312)