Asset Retirement Obligations - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Asset Retirement Obligation Liabilities [Line Items]
Cumulative asset retirement costs	$ 2.8	$ 2.8
Minimum [Member]
Asset Retirement Obligation Liabilities [Line Items]
Estimated lives for depreciation of asset retirement cost additions	51 months
Maximum [Member]
Asset Retirement Obligation Liabilities [Line Items]
Estimated lives for depreciation of asset retirement cost additions	57 months