Revenue from Contracts with Customers (Disaggregation of Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019		Jun. 30, 2018		Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	$ 646,114		$ 426,767	[1]	$ 1,236,664	$ 823,096	[1]
Gain (loss) on crude contracts	57,916	[2]	(37,624)	[1]	57,916	(67,126)	[1]
Gain on sale of assets and other income	5,019		437	[1]	5,473	8,400	[1]
Total revenues	709,049		389,580	[1]	1,300,053	764,370	[1]
Net crude oil and condensate revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	597,484		369,958		1,121,139	699,744
Net natural gas liquids revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	12,143		19,603		25,918	36,845
Net natural gas revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	36,487		37,206		89,607	86,507
United States | Net crude oil and condensate revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	192,182		198,823		325,772	381,472
United States | Net natural gas liquids revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	6,384		13,236		12,525	25,370
United States | Net natural gas revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	5,533		6,292		11,397	13,062
United States Offshore | Net crude oil and condensate revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	334,189		94,393		650,212	165,922
United States Offshore | Net natural gas liquids revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	2,988		2,920		7,164	4,559
United States Offshore | Net natural gas revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	6,643		2,825		9,149	5,762
Canada | Net crude oil and condensate revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	26,472		28,426		53,816	49,719
Canada | Net natural gas liquids revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	2,771		3,447		6,229	6,916
Canada | Net natural gas revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	24,311		28,089		69,061	67,683
Canada Offshore | Net crude oil and condensate revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	41,518		48,316		85,364	102,631
Other | Net crude oil and condensate revenue
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	$ 3,123		$ 0		$ 5,975	$ 0

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

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