Net Costs Incurred on Unevaluated Properties (Detail) (Evaluated Costs, USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Evaluated Costs
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Acquisition costs evaluated during the year	$ 9,739	$ (2,397)	$ 42,664
Exploration costs evaluated during the year	(1,209)	(51,207)	10,491
Capitalized interest costs evaluated during the year	37,656	42,033	30,783
Total costs incurred during the year, net of divestitures	$ 46,186	$ (11,571)	$ 83,938