Asset Retirement Obligations - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended
Jun. 30, 2014	Dec. 31, 2013
Asset Retirement Obligation Liabilities [Line Items]
Cumulative asset retirement costs	$ 3.9	$ 3.8
Increase in asset retirement costs	$ 0.1
Maximum [Member]
Asset Retirement Obligation Liabilities [Line Items]
Estimated lives for depreciation of asset retirement cost additions	54 months

Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

No definition available.

Name:	us-gaap_PropertyPlantAndEquipmentUsefulLife
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration