Risk Management and Hedging Activities - Derivative Contract Volumes (Details) gal in Millions, MWh in Millions, Dth in Millions, $ in Millions	Jun. 30, 2017 USD ($) gal Dth MWh	Dec. 31, 2016 USD ($) gal Dth MWh
Electricity purchases (in megawatt hours) [Member] | Commodity derivative [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, nonmonetary notional amount | MWh	11	5
Natural gas purchases (in decatherms) [Member] | Commodity derivative [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, nonmonetary notional amount | Dth	279	271
Fuel purchases (in gallons) [Member] | Commodity derivative [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, nonmonetary notional amount | gal	5	11
Interest rate swap [Member] | Interest Rate Contract [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, notional amount	$ (694)	$ (714)
Mortgage sale commitments, net [Member] | Interest Rate Contract [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, notional amount	$ (348)	$ (309)