Revenue from Contracts with Customers (Disaggregation of Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	$ 285,745	$ 680,436	$ 886,303	$ 1,309,790
(Loss) gain on crude contracts	(75,880)	57,916	324,792	57,916
Gain on sale of assets and other income	1,677	5,598	4,175	6,790
Total revenues and other income	211,542	743,950	1,215,270	1,374,496
Net crude oil and condensate revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	227,349	619,355	755,418	1,170,411
Net natural gas liquids revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	8,616	15,972	21,179	32,946
Net natural gas revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	49,780	45,109	109,706	106,433
United States | Net crude oil and condensate revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	54,550	193,565	185,786	328,241
United States | Net natural gas liquids revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	3,876	8,719	9,379	16,940
United States | Net natural gas revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	4,090	7,340	9,648	14,914
United States Offshore | Net crude oil and condensate revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	150,253	352,281	497,225	691,944
United States Offshore | Net natural gas liquids revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	3,464	4,478	8,490	9,770
United States Offshore | Net natural gas revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	10,665	9,219	25,660	13,696
Canada     | Net crude oil and condensate revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	11,527	28,031	34,910	56,972
Canada     | Net natural gas liquids revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	1,276	2,775	3,310	6,236
Canada     | Net natural gas revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	35,025	28,550	74,398	77,823
Canada Offshore | Net crude oil and condensate revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	11,077	42,355	35,691	87,279
Other | Net crude oil and condensate revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	$ (58)	$ 3,123	$ 1,806	$ 5,975