Asset Retirement Obligations - Changes in Asset Retirement Obligation Liabilities (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at January 1, 2018	$ 7,409
Accretion	135	$ 140
Amounts paid	(13)
Increases	15
Reclassifications	0
Balance at March 31, 2018	7,546
Short-Term Portion
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at January 1, 2018	235
Accretion	(22)
Amounts paid	(13)
Increases	0
Reclassifications	80
Balance at March 31, 2018	280
Long-Term Portion
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at January 1, 2018	7,174
Accretion	157
Amounts paid	0
Increases	15
Reclassifications	(80)
Balance at March 31, 2018	$ 7,266