Asset Retirement Obligations (Tables)

12 Months Ended

Dec. 31, 2011

Reconciliation of Beginning and Ending Aggregate Carrying Amount of Asset Retirement Obligation

reconciliation of the beginning and ending aggregate carrying amount of the asset retirement obligation for 2011 and 2010 is shown in the following table.

(Thousands of dollars)	2011			2010
Balance at beginning of year	$	555,248			476,938
Accretion expense		37,701			31,857
Liabilities incurred		51,858			59,605
Revision of previous estimates		7,608			14,170
Liabilities settled		(32,088	)*		(36,506	)
Changes due to translation of foreign currencies		(4,782	)		9,184
Balance at end of year	$	615,545			555,248

* Includes noncash settlements related to sale of assets in Spain in 2011.