Revenue from Contracts with Customers (Disaggregation of Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2019		Sep. 30, 2018		Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	$ 750,337		$ 475,458	[1]	$ 2,060,127	$ 1,330,399	[1]
Gain (loss) on crude contracts	63,247	[2]	(2,223)	[1]	121,163	(69,349)	[1]
Gain on sale of assets and other income	3,493		17,276	[1]	10,283	26,713	[1]
Total revenues	817,077		490,511	[1]	2,191,573	1,287,763	[1]
Net crude oil and condensate revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	680,132		395,670		1,850,542	1,107,855
Net natural gas liquids revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	16,722		26,936		49,668	68,690
Net natural gas revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	53,483		52,852		159,917	153,854
United States | Net crude oil and condensate revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	219,515		227,022		547,756	612,194
United States | Net natural gas liquids revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	5,557		19,196		22,497	48,615
United States | Net natural gas revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	5,848		8,833		20,762	25,670
United States Offshore | Net crude oil and condensate revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	398,518		95,059		1,090,462	264,174
United States Offshore | Net natural gas liquids revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	8,414		3,600		18,184	9,013
United States Offshore | Net natural gas revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	15,879		3,965		29,575	11,161
Canada | Net crude oil and condensate revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	31,758		34,504		88,730	87,018
Canada | Net natural gas liquids revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	2,751		4,140		8,987	11,062
Canada | Net natural gas revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	31,756		40,054		109,580	117,023
Canada Offshore | Net crude oil and condensate revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	28,408		35,929		115,686	141,313
Other | Net crude oil and condensate revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	$ 1,933		$ 3,156		$ 7,908	$ 3,156

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

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