Derivative Instruments (Offsetting Derivative Liabilities) (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
Derivative [Line Items]
Affiliate Balances, Offsetting Derivative Liabilities, Gross Amounts of Recognized Liabilities	$ (19,700)	$ (24,100)
Affiliate Balances, Offsetting Derivative Liabilities, Gross Amounts Offset in the Balance Sheets	9,300	21,100
Affiliate Balances, Offsetting Derivative Liabilities, Net Amount of Liabilities Presented in the Balance Sheets	(10,400)	(3,000)
Put premiums payable-affiliate
Derivative [Line Items]
Affiliate Balances, Offsetting Derivative Liabilities, Gross Amounts of Recognized Liabilities	(6,100)	(6,000)
Affiliate Balances, Offsetting Derivative Liabilities, Gross Amounts Offset in the Balance Sheets	6,100	6,000
Long-term put premiums payable-affiliate
Derivative [Line Items]
Affiliate Balances, Offsetting Derivative Liabilities, Gross Amounts of Recognized Liabilities	(13,600)	(18,100)
Affiliate Balances, Offsetting Derivative Liabilities, Gross Amounts Offset in the Balance Sheets	3,200	15,100
Affiliate Balances, Offsetting Derivative Liabilities, Net Amount of Liabilities Presented in the Balance Sheets	$ (10,400)	$ (3,000)