Price Risk Management Counterparties Representing 10% or More (Details)	Mar. 31, 2012	Dec. 31, 2011
Assets from price risk management activities:
Counterparty A	16.00%	16.00%
Counterparty B	12.00%	19.00%
Counterparty C	11.00%	7.00%
Counterparty D	8.00%	13.00%
Concentration of Risk, Derivative Instruments, Assets	47.00%	55.00%
Liabilities from price risk management activities:
Counterparty C	22.00%	23.00%
Counterparty E	13.00%	10.00%
Concentration of Risk, Derivative Instruments, Liabilities	35.00%	33.00%