Supplemental Information on Natural Gas and Oil Exploration, Development and Production Activities (Unaudited) - Costs Incurred for Property Acquisition, Exploration and Development (Parenthetical) (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2014 USD ($)
Gas Gathering Assets
Costs Incurred Oil And Gas Property Acquisition Exploration And Development Activities [Line Items]
Acquisition Costs, Period Cost	$ 134.8