Asset Retirement Obligations - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Asset Retirement Obligation Liabilities [Line Items]
Cumulative asset retirement costs	$ 1.8	$ 1.7
Increase in asset retirement costs	$ 0.1
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