Annual report pursuant to Section 13 and 15(d)

Reconciliation of Asset Retirement Obligations which are Included in Other Liabilities (Detail)

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Reconciliation of Asset Retirement Obligations which are Included in Other Liabilities (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Balance at beginning of period $ 3,286 $ 2,459
Changes in estimates 354 149
Liabilities incurred 335 118
Liabilities settled (8) (139)
Purchase of properties (385) (494)
Asset Retirement Obligation, Sale of Properties (310) 0
Accretion expense 272 205
Balance at end of period $ 4,314 $ 3,286