Asset Retirement Obligations - Changes in Asset Retirement Obligation Liabilities (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2015 USD ($)
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at January 1, 2015	$ 7,147
Accretion	331
Additions	28
Reclassifications	0
Amounts paid	(84)
Balance at June 30, 2015	7,422
Short-Term Portion [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at January 1, 2015	342
Accretion	62
Additions	0
Reclassifications	59
Amounts paid	(84)
Balance at June 30, 2015	379
Long-Term Portion [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at January 1, 2015	6,805
Accretion	269
Additions	28
Reclassifications	(59)
Amounts paid	0
Balance at June 30, 2015	$ 7,043