Customers from Whom We Derived Ten Percent or More of Total Oil and Gas Revenue (Detail)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Conoco, Inc.
Concentration Risk [Line Items]
Customers from whom 10% or more revenue derived	13.00%	28.00%	26.00%
Hess Corporation
Concentration Risk [Line Items]
Customers from whom 10% or more revenue derived			11.00%
Less than 10 %	Less than 10%	Less than 10%
Phillips Sixty Six
Concentration Risk [Line Items]
Customers from whom 10% or more revenue derived	18.00%
Less than 10 %		Less than 10%	Less than 10%
Sequent Energy Management LP
Concentration Risk [Line Items]
Customers from whom 10% or more revenue derived			10.00%
Less than 10 %	Less than 10%	Less than 10%
Shell Trading (US) Company
Concentration Risk [Line Items]
Customers from whom 10% or more revenue derived	41.00%	46.00%	40.00%