Price Risk Management Counterparties Representing 10% or More (Details)	Mar. 31, 2013	Dec. 31, 2012
Assets from price risk management activities:
Counterparty A	16.00%	3.00%
Counterparty B	15.00%	11.00%
Counterparty C	9.00%	13.00%
Counterparty D	9.00%	21.00%
Counterparty E	3.00%	10.00%
Concentration of Risk, Derivative Instruments, Assets	52.00%	58.00%
Liabilities from price risk management activities:
Counterparty F	20.00%	24.00%
Counterparty G	14.00%	0.00%
Counterparty H	10.00%	14.00%
Counterparty I	7.00%	10.00%
Concentration of Risk, Derivative Instruments, Liabilities	51.00%	48.00%