Fair Value Measurements (Assets and Liabilities Measured on Recurring Basis) (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Assets and liabilities measured at fair value on a recurring basis
Restricted cash			$ 1,033
Commodity derivatives, net	$ (33,095)	$ 536	(5,628)
Provisionally price contracts	(62,298)	(165,284)	(118,737)
Convertible preferred securities	7,488	7,388	3,294
Other assets and liabilities	4,099	8,461	7,997
Total	(83,806)	(148,899)	(112,041)
Level 1
Assets and liabilities measured at fair value on a recurring basis
Restricted cash			1,033
Commodity derivatives, net	15,356	18,603	2,817
Provisionally price contracts	(37,787)	(98,190)	(87,958)
Convertible preferred securities	0	0	0
Other assets and liabilities	4,136	9,705	10,155
Total	(18,295)	(69,882)	(73,953)
Level 2
Assets and liabilities measured at fair value on a recurring basis
Restricted cash			0
Commodity derivatives, net	(48,451)	(18,067)	(8,445)
Provisionally price contracts	(24,511)	(67,094)	(30,779)
Convertible preferred securities	0	0	0
Other assets and liabilities	(37)	(1,244)	(2,158)
Total	(72,999)	(86,405)	(41,382)
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	7,488	7,388	3,294
Other assets and liabilities	0	0	0
Total	$ 7,488	$ 7,388	$ 3,294