Derivative Instruments (Offsetting Derivative Assets) (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
Derivative [Line Items]
Affiliate Balances, Offsetting Derivative Assets, Gross Amounts of Recognized Assets	$ 30,800	$ 90,600
Affiliate Balances, Offsetting Derivative Assets, Gross Amounts Offset in the Balance Sheets	(9,300)	(21,100)
Affiliate Balances, Offsetting Derivative Assets, Net Amount of Assets Presented in the Balance Sheets	21,500	69,500
Accounts receivable monetized gains-affiliate
Derivative [Line Items]
Affiliate Balances, Offsetting Derivative Assets, Gross Amounts of Recognized Assets	27,600	75,500
Affiliate Balances, Offsetting Derivative Assets, Gross Amounts Offset in the Balance Sheets	(6,100)	(6,000)
Affiliate Balances, Offsetting Derivative Assets, Net Amount of Assets Presented in the Balance Sheets	21,500	69,500
Long-term receivable monetized gains-affiliate
Derivative [Line Items]
Affiliate Balances, Offsetting Derivative Assets, Gross Amounts of Recognized Assets	3,200	15,100
Affiliate Balances, Offsetting Derivative Assets, Gross Amounts Offset in the Balance Sheets	$ (3,200)	$ (15,100)