Concentrations - Customers from Whom We Derived 10% or More of Total Oil and Gas Revenue (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Conoco, Inc [Member]
Concentration Risk [Line Items]
Concentration risk, benchmark description	Less than 10%	Less than 10%
Customer Concentration Risk [Member] | Sales Revenue, Product Line [Member] | Conoco, Inc [Member]
Concentration Risk [Line Items]
Customers from whom 10% or more revenue derived			13.00%
Customer Concentration Risk [Member] | Sales Revenue, Product Line [Member] | Phillips 66 Company [Member]
Concentration Risk [Line Items]
Customers from whom 10% or more revenue derived	31.00%	35.00%	18.00%
Customer Concentration Risk [Member] | Sales Revenue, Product Line [Member] | Shell Trading (US) Company [Member]
Concentration Risk [Line Items]
Customers from whom 10% or more revenue derived	32.00%	33.00%	41.00%