Counterparties representing 10% or more (Details)	Dec. 31, 2016	Dec. 31, 2015
Assets from price risk management activities:
Counterparty A	22.00%	5.00%
Counterparty B	17.00%	8.00%
Counterparty C	12.00%	8.00%
Counterparty D	8.00%	10.00%
Counterparty E	1.00%	59.00%
Concentration of Risk, Derivative Instruments, Assets	60.00%	90.00%
Liabilities from price risk management activities: [Abstract]
Counterparty F	66.00%	36.00%
Counterparty C for PRM Liabs	7.00%	10.00%
Counterparty B for PRM Liabs	5.00%	10.00%
Concentration of Risk, Derivative Instruments, Liabilities	78.00%	56.00%