Asset Retirement Obligations - Changes in Asset Retirement Obligation Liabilities (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2015 USD ($)
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at January 1, 2015	$ 7,147
Accretion	497
Additions	45
Reclassifications	0
Amounts paid	(128)
Balance at September 30, 2015	7,561
Short-Term Portion [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at January 1, 2015	342
Accretion	93
Additions	0
Reclassifications	20
Amounts paid	(128)
Balance at September 30, 2015	327
Long-Term Portion [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at January 1, 2015	6,805
Accretion	404
Additions	45
Reclassifications	(20)
Amounts paid	0
Balance at September 30, 2015	$ 7,234