Fair Value Measurement (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Assets (liabilities) measured at fair value
Assets (liabilities), fair value	$ (18,563)	$ (41,582)
Level 1
Assets (liabilities) measured at fair value
Assets (liabilities), fair value	10	(3,742)
Level 2
Assets (liabilities) measured at fair value
Assets (liabilities), fair value	1,492	(3,447)
Level 3
Assets (liabilities) measured at fair value
Assets (liabilities), fair value	(20,065)	(34,393)
Commodity contract derivatives
Assets (liabilities) measured at fair value
Assets (liabilities), fair value	1,502	(7,189)
Commodity contract derivatives | Level 1
Assets (liabilities) measured at fair value
Assets (liabilities), fair value	10	(3,742)
Commodity contract derivatives | Level 2
Assets (liabilities) measured at fair value
Assets (liabilities), fair value	1,492	(3,447)
Commodity contract derivatives | Level 3
Assets (liabilities) measured at fair value
Assets (liabilities), fair value	0	0
Contingent considerations for acquisitions
Assets (liabilities) measured at fair value
Assets (liabilities), fair value	(20,065)	(34,393)
Contingent considerations for acquisitions | Level 1
Assets (liabilities) measured at fair value
Assets (liabilities), fair value	0	0
Contingent considerations for acquisitions | Level 2
Assets (liabilities) measured at fair value
Assets (liabilities), fair value	0	0
Contingent considerations for acquisitions | Level 3
Assets (liabilities) measured at fair value
Assets (liabilities), fair value	$ (20,065)	$ (34,393)