Suspended Exploratory Well Costs - Aging of Capitalized Exploratory Well Costs Suspended more than One Year (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Exploratory Well Costs Capitalized In Year Two Thousand And Thirteen	Mar. 31, 2012 Exploratory Well Costs Capitalized In Year Two Thousand And Twelve	Mar. 31, 2011 Exploratory Well Costs Capitalized In Year Two Thousand And Eleven	Mar. 31, 2010 Exploratory Well Costs Capitalized In Year Two Thousand And Ten	Mar. 31, 2009 Exploratory Well Costs Capitalized In Year Two Thousand And Nine	Mar. 31, 2008 Exploratory Well Costs Capitalized In Year Two Thousand And Eight
Aging Of Capitalized Exploratory Well Costs [Line Items]
Capitalized exploratory well costs that have been capitalized for more than one year	$ 22,259	$ 11,395	$ 5,854	$ 5,961	$ 5,965	$ 72	$ 2,884	$ 1,523