Asset Retirement Obligations - Changes in Asset Retirement Obligation Liabilities (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at January 1, 2017	$ 7,557
Accretion	140	$ 155
Amounts paid	(44)
Reclassifications	0
Balance at March 31, 2017	7,653
Short-Term Portion
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at January 1, 2017	85
Accretion	2
Amounts paid	(44)
Reclassifications	162
Balance at March 31, 2017	205
Long-Term Portion
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at January 1, 2017	7,472
Accretion	138
Amounts paid	0
Reclassifications	(162)
Balance at March 31, 2017	$ 7,448