Counterparties representing 10% or more (Details)	Dec. 31, 2017	Dec. 31, 2016
Assets from price risk management activities:
Counterparty A	39.00%	22.00%
Counterparty B	12.00%	17.00%
Counterparty C	3.00%	12.00%
Concentration of Risk, Derivative Instruments, Assets	54.00%	51.00%
Liabilities from price risk management activities: [Abstract]
Counterparty D	62.00%	66.00%
Concentration of Risk, Derivative Instruments, Liabilities	62.00%	66.00%