Fair Value Measurements (Assets and Liabilities Measured on Recurring Basis) (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Assets and liabilities measured at fair value on a recurring basis
Commodity derivatives, net	$ (16,324)	$ 536	$ 5,967
Provisionally price contracts	(78,833)	(165,284)	(119,490)
Convertible preferred securities	7,154	7,388	6,638
Other assets and liabilities	6,181	8,461	9,067
Total	(81,822)	(148,899)	(97,818)
Level 1
Assets and liabilities measured at fair value on a recurring basis
Commodity derivatives, net	51,595	18,603	26,738
Provisionally price contracts	(55,697)	(98,190)	(85,546)
Convertible preferred securities	0	0	0
Other assets and liabilities	5,988	9,705	10,996
Total	1,886	(69,882)	(47,812)
Level 2
Assets and liabilities measured at fair value on a recurring basis
Commodity derivatives, net	(67,919)	(18,067)	(20,771)
Provisionally price contracts	(23,136)	(67,094)	(33,944)
Convertible preferred securities	0	0	0
Other assets and liabilities	193	(1,244)	(1,929)
Total	(90,862)	(86,405)	(56,644)
Level 3
Assets and liabilities measured at fair value on a recurring basis
Commodity derivatives, net	0	0	0
Provisionally price contracts	0	0	0
Convertible preferred securities	7,154	7,388	6,638
Other assets and liabilities	0	0	0
Total	$ 7,154	$ 7,388	$ 6,638