Price Risk Management Counterparties Representing 10% or More (Details)	Sep. 30, 2013	Dec. 31, 2012
Assets from price risk management activities:
Counterparty A	21.00%	0.00%
Counterparty B	14.00%	0.00%
Counterparty C	11.00%	0.00%
Counterparty D	9.00%	21.00%
Counterparty E	4.00%	11.00%
Counterparty F	1.00%	13.00%
Counterparty G	0.00%	10.00%
Concentration of Risk, Derivative Instruments, Assets	60.00%	55.00%
Liabilities from price risk management activities:
Counterparty H	16.00%	0.00%
Counterparty I	14.00%	24.00%
Counterparty A	10.00%	14.00%
Counterparty E	10.00%	8.00%
Counterparty J	9.00%	10.00%
Concentration of Risk, Derivative Instruments, Liabilities	59.00%	56.00%