Asset Retirement Obligations - Changes in Asset Retirement Obligation Liabilities (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2016 USD ($)
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at January 1, 2016	$ 7,839
Accretion	155
Amounts Paid	(59)
Asset additions and (reductions) recorded	14
Reclassifications	0
Balance at March 31, 2016	7,949
Short-Term Portion [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at January 1, 2016	296
Accretion	9
Amounts Paid	(59)
Asset additions and (reductions) recorded	0
Reclassifications	38
Balance at March 31, 2016	284
Long-Term Portion [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at January 1, 2016	7,543
Accretion	146
Amounts Paid	0
Asset additions and (reductions) recorded	14
Reclassifications	(38)
Balance at March 31, 2016	$ 7,665