Properties and Equipment (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($) Wells	Dec. 31, 2018 USD ($) Wells	Dec. 31, 2017 USD ($)
Property, Plant and Equipment [Line Items]
Document Period End Date	Mar. 31, 2019
Reclassification to Well, Facilities, and Equipment Based on Determination of Proved Reserves	$ 0	$ (38,387)
Proved Natural Gas and Crude Oil Properties	5,681,685	5,452,613
Unproved Natural Gas and Crude Oil Properties	485,571	492,594
Total Natural Gas and Crude Oil Properties	6,167,256	5,945,207
Equipment and other	59,510	60,612
Land and Buildings	12,497	11,243
Construction in Progress	404,229	356,095
Properties and equipment, at cost	6,643,492	6,373,157
Accumulated DD&A	(2,521,843)	(2,370,295)
Property, Plant and Equipment, Net	4,121,649	4,002,862
Capitalized Exploratory Well Cost, Additions Pending Determination of Proved Reserves	12,853	35,127
Capitalized Exploratory Well Costs	$ 25,041	$ 12,188	$ 15,448
Wells to be completed | Wells	2	2