Acquisition (Narrative) (Details) $ in Thousands	1 Months Ended	3 Months Ended												12 Months Ended
	Dec. 31, 2018 USD ($) Boe / d MMBoe	Dec. 31, 2018 USD ($) MMBoe		Sep. 30, 2018 USD ($)	[1]	Jun. 30, 2018 USD ($)	[1]	Mar. 31, 2018 USD ($)	[1]	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) MMBoe	Dec. 31, 2017 USD ($)	[2]	Dec. 31, 2016 USD ($)	[2]
Business Acquisition [Line Items]
Revenue from sales to customers		$ 664,700	[1]	$ 659,800		$ 655,200		$ 606,900		$ 580,500	$ 511,200	$ 477,600	$ 509,000	$ 2,586,627	$ 2,078,548		$ 1,862,891
Income (loss) from continuing operations before income taxes		$ 106,200	[1]	$ 146,800		$ 82,300		$ 97,000		$ 2,400	$ (63,600)	$ (21,900)	$ 154,900	$ 432,338	$ 71,802		$ (493,115)
Petrobas America Inc [Member]
Business Acquisition [Line Items]
Production capacity | Boe / d	50,000
Proved reserves | MMBoe	97	97												97
Acquisition, cash paid	$ 794,623
Percentage of interest in subsidiary transferred	20.00%
Revenue from sales to customers	$ 56,700
Income (loss) from continuing operations before income taxes	36,500
Acquisition-related costs														$ 6,200
Petrobas America Inc [Member] | Sales Thresholds 2019 to 2025 [Member]
Business Acquisition [Line Items]
Contingent consideration, maximum	$ 150,000	$ 150,000												$ 150,000
MP Gulf Of Mexico LLC [Member]
Business Acquisition [Line Items]
Parent ownership interest	80.00%	80.00%												80.00%
Petrobas America Inc [Member] | MP Gulf Of Mexico LLC [Member]
Business Acquisition [Line Items]
Noncontrolling ownership interest	20.00%	20.00%												20.00%

[1] 2018 includes noncontrolling interest in MP GOM.

[2] Reclassified to conform to current presentation (see Note B).