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