Year Ended December 31,

2011

2010

2009

Asset retirement obligations as of the beginning of the year, including current portion

$

373,920

$

320,599

$

275,628

Liabilities incurred

7,993

1,528

3,035

Liabilities settled

(63,226

)

(36,392

)

(67,858

)

Liabilities assumed

59,441

—

Divestment of properties

(10,900

)

(692

)

(5,941

)

Accretion expense

30,764

34,469

39,306

Revision of estimates

27,787

54,408

76,429

Asset retirement obligations as of the end of the year, including current portion

$

425,779

$

373,920

$

320,599