Commitments and Contingencies (Marcellus Acquisition) (Details) (USD $)	9 Months Ended		0 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Dec. 14, 2011 SM Energy Company [Member]	Jul. 17, 2011 SM Energy Company [Member]	Sep. 30, 2013 SM Energy Company [Member]
Business Combination, Separately Recognized Transactions [Line Items]
SM Purchase Agreement entry date					Jul. 17, 2011
Payments to acquire oil and gas property	$ 2,602,000	$ 228,437,000		$ 110,000,000
Termination date					December 14, 2011
Valuation of defects threshold percentage			15.00%
Applicable asset group purchase price			$ 85,000,000