Fair Value Measurements (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Assets and liabilities measured at fair value on a recurring basis
Restricted cash			$ 752
Commodity derivatives, net	$ 28,711	$ 536	18,024
Provisionally price contracts	(80,325)	(165,284)	(123,957)
Convertible preferred securities	7,388	7,388	3,294
Other assets and liabilities	8,493	8,461	6,377
Total	(35,733)	(148,899)	(95,510)
Level 1
Assets and liabilities measured at fair value on a recurring basis
Restricted cash			752
Commodity derivatives, net	35,888	18,603	29,566
Provisionally price contracts	(48,478)	(98,190)	(86,314)
Convertible preferred securities	0	0	0
Other assets and liabilities	8,947	9,705	8,518
Total	(3,643)	(69,882)	(47,478)
Level 2
Assets and liabilities measured at fair value on a recurring basis
Restricted cash			0
Commodity derivatives, net	(7,177)	(18,067)	(11,542)
Provisionally price contracts	(31,847)	(67,094)	(37,643)
Convertible preferred securities	0	0	0
Other assets and liabilities	(454)	(1,244)	(2,141)
Total	(39,478)	(86,405)	(51,326)
Level 3
Assets and liabilities measured at fair value on a recurring basis
Restricted cash			0
Commodity derivatives, net	0	0	0
Provisionally price contracts	0	0	0
Convertible preferred securities	7,388	7,388	3,294
Other assets and liabilities	0	0	0
Total	$ 7,388	$ 7,388	$ 3,294