Suspended Exploratory Well Costs - Aging of Capitalized Exploratory Well Costs Suspended more than One Year (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 Exploratory Well Costs Capitalized In Year Two Thousand And Thirteen	Dec. 31, 2012 Exploratory Well Costs Capitalized In Year Two Thousand And Twelve	Dec. 31, 2011 Exploratory Well Costs Capitalized In Year Two Thousand And Eleven	Dec. 31, 2010 Exploratory Well Costs Capitalized In Year Two Thousand And Ten	Dec. 31, 2009 Exploratory Well Costs Capitalized In Year Two Thousand And Nine	Dec. 31, 2008 Exploratory Well Costs Capitalized In Year Two Thousand And Eight
Projects With Exploratory Well Costs Capitalized For More Than One Year [Line Items]
Capitalized exploratory well costs that have been capitalized for more than one year	$ 12,880	$ 11,395	$ 208	$ 6,904	$ 1,289	$ 72	$ 2,884	$ 1,523