Derivative and Hedging Activities - Quantitative Disclosures Level 3 Table (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($)
Energy Related Derivative, Oil Trades [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted Average Of Price Volatility Curve Percentage	23.95%
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value	$ (1)
Energy Related Derivative, Oil Trades [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted Average Of Price Volatility Curve Percentage	17.23%
Energy Related Derivative, Oil Trades [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted Average Of Price Volatility Curve Percentage	25.95%
Energy Related Derivative, Natural Gas Trades [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted Average Of Price Volatility Curve Percentage	30.71%
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value	$ (9)
Energy Related Derivative, Natural Gas Trades [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted Average Of Price Volatility Curve Percentage	19.72%
Energy Related Derivative, Natural Gas Trades [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted Average Of Price Volatility Curve Percentage	68.72%