Summary of Significant Accounting Policies (Schedule of Impact of Adoption) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Crude oil revenues	$ 229,798	$ 142,806	$ 424,717	$ 270,898
Natural gas liquids revenues	21,269	7,786	38,171	15,211
Natural gas revenues	12,906	15,891	26,365	31,729
Oil and Gas Sales Revenue	263,973	166,483	489,253	317,838
Lease operating	35,151	36,048	74,424	65,893
Income before income taxes	35,792	$ 56,306	63,603	$ 96,327
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Crude oil revenues	229,658		424,452
Natural gas liquids revenues	20,139		36,235
Natural gas revenues	12,272		25,159
Oil and Gas Sales Revenue	262,069		485,846
Lease operating	33,247		71,017
Income before income taxes	35,792		63,603
Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Crude oil revenues	140		265
Natural gas liquids revenues	1,130		1,936
Natural gas revenues	634		1,206
Oil and Gas Sales Revenue	1,904		3,407
Lease operating	1,904		3,407
Income before income taxes	$ 0		$ 0
Crude Oil
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Percent Difference Between Revenue Guidance In Effect Before And After Topic 606	0.10%		0.10%
Natural Gas Liquids
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Percent Difference Between Revenue Guidance In Effect Before And After Topic 606	5.60%		5.30%
Natural Gas
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Percent Difference Between Revenue Guidance In Effect Before And After Topic 606	5.20%		4.80%
Total Oil and Gas
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Percent Difference Between Revenue Guidance In Effect Before And After Topic 606	0.70%		0.70%
Lease Operating Expense
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Percent Difference Between Revenue Guidance In Effect Before And After Topic 606	5.70%		4.80%
Income Before Income Taxes
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Percent Difference Between Revenue Guidance In Effect Before And After Topic 606	0.00%		0.00%