Acquisition (Narrative) (Details) $ in Thousands	1 Months Ended	3 Months Ended
	Dec. 31, 2018 USD ($) Boe / d MMBoe	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	[1]
Business Acquisition [Line Items]
Revenue from sales to customers		$ 590,550	$ 396,329
Income (loss) from continuing operations before income taxes		33,745	$ (21,058)
Petrobas America Inc [Member]
Business Acquisition [Line Items]
Production capacity | Boe / d	50,000
Proved reserves | MMBoe	97
Acquisition, cash paid	$ 788,724
Percentage of interest in subsidiary transferred	20.00%
Revenue from sales to customers		234,000
Income (loss) from continuing operations before income taxes		$ 147,700
Petrobas America Inc [Member] | Sales Thresholds 2019 to 2022 [Member]
Business Acquisition [Line Items]
Contingent consideration, maximum	$ 150,000
MP Gulf Of Mexico LLC [Member]
Business Acquisition [Line Items]
Parent ownership interest	80.00%
Petrobas America Inc [Member] | MP Gulf Of Mexico LLC [Member]
Business Acquisition [Line Items]
Noncontrolling ownership interest	20.00%

[1] Reclassified to conform to current presentation (see Note A).