Revenue from Contracts with Customers (Disaggregation of Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	$ 425,324	$ 750,337	$ 1,311,627	$ 2,060,127
(Loss) gain on crude contracts	(5,290)	63,247	319,502	121,163
Gain on sale of assets and other income	1,831	3,493	6,006	10,283
Total revenues and other income	421,865	817,077	1,637,135	2,191,573
Net crude oil and condensate revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	354,947	680,131	1,110,365	1,850,542
Net natural gas liquids revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	14,311	16,722	35,490	49,668
Net natural gas revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	56,066	53,484	165,772	159,917
United States | Net crude oil and condensate revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	86,498	219,515	272,284	547,756
United States | Net natural gas liquids revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	6,766	5,557	16,145	22,497
United States | Net natural gas revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	4,529	5,848	14,177	20,762
United States Offshore | Net crude oil and condensate revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	216,918	398,518	714,143	1,090,462
United States Offshore | Net natural gas liquids revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	4,765	8,414	13,255	18,184
United States Offshore | Net natural gas revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	9,827	15,879	35,487	29,575
Canada     | Net crude oil and condensate revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	32,358	31,758	67,268	88,730
Canada     | Net natural gas liquids revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	2,780	2,751	6,090	8,987
Canada     | Net natural gas revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	41,710	31,757	116,108	109,580
Canada Offshore | Net crude oil and condensate revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	19,173	28,407	54,864	115,686
Other | Net crude oil and condensate revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	$ 0	$ 1,933	$ 1,806	$ 7,908