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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2017
Asset Retirement Obligation Disclosure [Abstract]  
Asset Retirement Obligations
Activity related to the Company's asset retirement obligations is as follows:
(In thousands)
 
Year Ended   December 31, 2017
Balance at beginning of period (1)
 
$
133,733

Liabilities incurred
 
4,653

Liabilities settled
 
(1,293
)
Liabilities divested
 
(77,965
)
Liabilities transferred to liabilities held for sale
 
(15,748
)
Accretion expense
 
5,173

Balance at end of period (2)
 
$
48,553