Fair Value Measurements (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Assets and liabilities measured at fair value on a recurring basis
Restricted cash		$ 471	$ 190
Commodity derivatives, net	$ 5,967	22,041	(541)
Provisionally price contracts	(119,490)	(170,197)	(99,522)
Convertible preferred securities	6,638	3,294	3,294
Other assets and liabilities	9,067	6,861	6,241
Total	(97,818)	(137,530)	(90,338)
Level 1
Assets and liabilities measured at fair value on a recurring basis
Restricted cash		471	190
Commodity derivatives, net	26,738	29,872	34,620
Provisionally price contracts	(85,546)	(105,321)	(79,022)
Convertible preferred securities	0	0	0
Other assets and liabilities	10,996	9,391	11,015
Total	(47,812)	(65,587)	(33,197)
Level 2
Assets and liabilities measured at fair value on a recurring basis
Restricted cash		0	0
Commodity derivatives, net	(20,771)	(7,831)	(35,161)
Provisionally price contracts	(33,944)	(64,876)	(20,500)
Convertible preferred securities	0	0	0
Other assets and liabilities	(1,929)	(2,530)	(4,774)
Total	(56,644)	(75,237)	(60,435)
Level 3
Assets and liabilities measured at fair value on a recurring basis
Restricted cash		0	0
Commodity derivatives, net	0	0	0
Provisionally price contracts	0	0
Convertible preferred securities	6,638	3,294	3,294
Other assets and liabilities	0	0	0
Total	$ 6,638	$ 3,294	$ 3,294