Price Risk Management Counterparties Representing 10% or More (Details)	Jun. 30, 2013	Dec. 31, 2012
Assets from price risk management activities:
Counterparty A	16.00%	0.00%
Counterparty B	13.00%	6.00%
Counterparty C	10.00%	0.00%
Counterparty D	4.00%	11.00%
Counterparty E	4.00%	21.00%
Counterparty F	2.00%	13.00%
Counterparty G	1.00%	10.00%
Concentration of Risk, Derivative Instruments, Assets	50.00%	61.00%
Liabilities from price risk management activities:
Counterparty H	16.00%	24.00%
Counterparty I	15.00%	0.00%
Counterparty A	10.00%	14.00%
Counterparty J	8.00%	10.00%
Concentration of Risk, Derivative Instruments, Liabilities	49.00%	48.00%