Risk Management and Hedging Activities - Balance Sheet Location (Details) MWh in Millions, Dth in Millions, $ in Millions		Jun. 30, 2015 USD ($) MWh Dth	Dec. 31, 2014 USD ($) MWh Dth
Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative, Net Liability Position, Aggregate Fair Value		$ 5	$ 4
Commodity Contract [Member] | Not Designated as Hedging Instrument [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement	[1]	12	9
Commodity Contract [Member] | Not Designated as Hedging Instrument [Member] | Other long-term liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement	[1]	$ 21	$ 21
Electricity purchases (sales), net, in megawatt hours [Member] | Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative, Nonmonetary Notional Amount | MWh		(3)	(3)
Natural gas purchases, in decatherms [Member] | Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative, Nonmonetary Notional Amount | Dth		131	115
Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement		$ 33	$ 30
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement		33	30
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Net Regulatory Asset (Liability), Unrealized Loss (Gain) On Derivative Contracts		33	30	[1]
Derivative Liability, Fair Value, Gross Liability	[1]	$ 33	$ 30

[1] The Company's commodity derivatives not designated as hedging contracts will be included in regulated rates when settled and as of June 30, 2015 and December 31, 2014, a regulatory asset of $33 million and $30 million, respectively, was recorded related to the derivative liability of $33 million and $30 million, respectively.