Leases (Operating Leases) (Details) - drilling_rig	Jun. 30, 2019	Dec. 31, 2018	Dec. 31, 2016	Dec. 31, 2014
Lessee, Lease, Description [Line Items]
Number of drilling rigs		2
CO2 Compressor
Lessee, Lease, Description [Line Items]
Operating lease, term	7 years		84 months	84 months
Drilling Rigs
Lessee, Lease, Description [Line Items]
Operating lease, term	15 months	15 months