Fair Value Measurement - (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets (liabilities) measured at fair value
Assets (liabilities), fair value	$ (41,582)	$ (75,907)
Level 1
Assets (liabilities) measured at fair value
Assets (liabilities), fair value	(3,742)	(1,297)
Level 2
Assets (liabilities) measured at fair value
Assets (liabilities), fair value	(3,447)	(28,042)
Level 3
Assets (liabilities) measured at fair value
Assets (liabilities), fair value	(34,393)	(46,568)
Commodity contract derivatives
Assets (liabilities) measured at fair value
Assets (liabilities), fair value	(7,189)	(2,239)
Commodity contract derivatives | Level 1
Assets (liabilities) measured at fair value
Assets (liabilities), fair value	(3,742)	(1,297)
Commodity contract derivatives | Level 2
Assets (liabilities) measured at fair value
Assets (liabilities), fair value	(3,447)	(942)
Commodity contract derivatives | Level 3
Assets (liabilities) measured at fair value
Assets (liabilities), fair value	0	0
Convertible debt conversion liability
Assets (liabilities) measured at fair value
Assets (liabilities), fair value		(27,100)
Convertible debt conversion liability | Level 1
Assets (liabilities) measured at fair value
Assets (liabilities), fair value		0
Convertible debt conversion liability | Level 2
Assets (liabilities) measured at fair value
Assets (liabilities), fair value		(27,100)
Convertible debt conversion liability | Level 3
Assets (liabilities) measured at fair value
Assets (liabilities), fair value		0
Contingent consideration for acquisitions
Assets (liabilities) measured at fair value
Assets (liabilities), fair value	(34,393)	(46,568)
Contingent consideration for acquisitions | Level 1
Assets (liabilities) measured at fair value
Assets (liabilities), fair value	0	0
Contingent consideration for acquisitions | Level 2
Assets (liabilities) measured at fair value
Assets (liabilities), fair value	0	0
Contingent consideration for acquisitions | Level 3
Assets (liabilities) measured at fair value
Assets (liabilities), fair value	$ (34,393)	$ (46,568)