Risk Management and Hedging Activities - Commodity derivative assets and liabilities offsetting (Details) - Commodity Derivatives - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Derivative Assets And Liabilities Offsetting Table Text Block [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 0	$ 104
Derivative Asset, Fair Value, Amount Offset Against Liabilities	0	63
Net Amounts Presented in the Condensed Consolidated Balance Sheet, Assets	0	41
Derivative Liability, Fair Value, Gross Liability	0	63
Derivative Liability, Fair Value, Amount Offset Against Assets	0	63
Net Amounts Presented in the Condensed Consolidated Balance Sheets, Liabilities	$ 0	$ 0