Fair Value Measurements (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents			$ 11,578
Restricted cash	$ 1,033	$ 471	987
Commodity derivatives, net	(5,628)	22,041	72,495
Provisionally price contracts	(118,737)	(170,197)	(60,708)
Convertible preferred securities	3,294	3,294	3,294
Other assets and liabilities	7,997	6,861	654
Total	(112,041)	(137,530)	28,300
Level 1
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents			11,578
Restricted cash	1,033	471	987
Commodity derivatives, net	2,817	29,872	48,412
Provisionally price contracts	(87,958)	(105,321)	(42,213)
Convertible preferred securities	0	0	0
Other assets and liabilities	10,155	9,391	6,080
Total	(73,953)	(65,587)	24,844
Level 2
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents			0
Restricted cash	0	0	0
Commodity derivatives, net	(8,445)	(7,831)	24,083
Provisionally price contracts	(30,779)	(64,876)	(18,495)
Convertible preferred securities	0	0	0
Other assets and liabilities	(2,158)	(2,530)	(5,426)
Total	(41,382)	(75,237)	162
Level 3
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents			0
Restricted cash	0	0	0
Commodity derivatives, net	0	0	0
Provisionally price contracts	0	0	0
Convertible preferred securities	3,294	3,294	3,294
Other assets and liabilities	0	0	0
Total	$ 3,294	$ 3,294	$ 3,294