Derivative Instruments (Narrative) (Details) - USD ($)	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Derivative [Line Items]
Net derivative assets	$ 14,000	$ 15,700
Accumulated other comprehensive income	1,400	3,900
Current portion of put premiums payable-affiliate	6,100	5,500
Long-term put premiums payable-affiliate	1,600	$ 6,300
Other Comprehensive Income
Derivative [Line Items]
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months	1,300
Net Deferred Gain (Loss) to be Reclassified into Net Income in Later Periods	100
Allocation To Limited Partner Only | Other Comprehensive Income
Derivative [Line Items]
Net Derivative Gains (Losses) Limited Partner	$ 2,100