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Asset retirement obligations
 12 Months Ended
Dec. 31, 2017
Asset Retirement Obligation [Abstract]  
Asset retirement obligations Asset Retirement Obligations
The Company records obligations related to retirement costs of natural gas distribution mains and lines, natural gas transmission lines, storage facilities, decommissioning of certain electric generating facilities, reclamation of certain aggregate properties, special handling and disposal of hazardous materials at certain electric generating facilities, natural gas distribution facilities and buildings, and certain other obligations as asset retirement obligations.
A reconciliation of the Company's liability, which is included in other accrued liabilities and deferred credits and other liabilities - other on the Consolidated Balance Sheets, for the years ended December 31 was as follows:
 
2017

2016

 
(In thousands)
Balance at beginning of year
$
314,970

$
242,224

Liabilities incurred
15,110

15,114

Liabilities settled
(4,981
)
(4,338
)
Accretion expense*
16,839

13,918

Revisions in estimates
31

48,052

Balance at end of year
$
341,969

$
314,970


*
Includes $15.6 million and $12.7 million in 2017 and 2016, respectively, related to regulatory assets.
 

The 2016 revisions in estimates consist principally of updated asset retirement obligation costs associated with natural gas transmission lines and storage facilities at the pipeline and midstream segment.
The Company believes that largely all expenses related to asset retirement obligations at the Company's regulated operations will be recovered in rates over time and, accordingly, defers such expenses as regulatory assets. For more information on the Company's regulatory assets and liabilities, see Note 4.