Derivative Financial Instruments (Settlement) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Derivatives
Gain (loss) on commodity derivative instruments	$ 16,241	$ 37,430	$ 33,897	$ (1,611)
Amount received on settlements of derivatives	2,600		92,000
Amount paid on settlements of derivatives		2,500		17,400
Other income (expenses)
Derivatives
Gain (loss) on commodity derivative instruments	$ 16,241	$ 37,430	$ 33,897	$ (1,611)