Asset Retirement Obligations - Additional Information (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2016	Dec. 31, 2015
Asset Retirement Obligation Liabilities [Line Items]
Cumulative asset retirement costs	$ 3.6	$ 3.6
Minimum [Member]
Asset Retirement Obligation Liabilities [Line Items]
Estimated lives for depreciation of asset retirement cost additions	54 months
Maximum [Member]
Asset Retirement Obligation Liabilities [Line Items]
Estimated lives for depreciation of asset retirement cost additions	60 months