Fair Value Measurements (Details 1) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents			$ 25,496
Restricted cash	$ 752	$ 471	718
Commodity derivatives, net	18,024	22,041	27,419
Provisionally price contracts	(123,957)	(170,197)	131,192
Convertible preferred securities	3,294	3,294	775
Other assets and liabilities	6,377	6,861	9,290
Total	(95,510)	(137,530)	194,890
Level 1
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents			25,496
Restricted cash	752	471	718
Commodity derivatives, net	29,566	29,872	38,070
Provisionally price contracts	(86,314)	(105,321)	88,356
Convertible preferred securities	0	0	0
Other assets and liabilities	8,518	9,391	13,958
Total	(47,478)	(65,587)	166,598
Level 2
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents			0
Restricted cash	0	0	0
Commodity derivatives, net	(11,542)	(7,831)	(10,651)
Provisionally price contracts	(37,643)	(64,876)	42,836
Convertible preferred securities	0	0	0
Other assets and liabilities	(2,141)	(2,530)	(4,828)
Total	(51,326)	(75,237)	27,357
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	775
Other assets and liabilities	0	0	160
Total	$ 3,294	$ 3,294	$ 935