Reconciliation of Beginning and Ending Aggregate Carrying Amount of Asset Retirement Obligation (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011		Dec. 31, 2010
Reconciliation of Changes in Asset Retirement Obligations [Line Items]
Accretion of asset retirement obligations	$ 39,341	$ 34,724	[1]	$ 29,580	[1]
U.K. exploration and production operations
Reconciliation of Changes in Asset Retirement Obligations [Line Items]
Accretion of asset retirement obligations		$ 2,977

[1] Reclassified to conform to current presentation.