Property, Plant and Equipment (Tables)

12 Months Ended

Dec. 31, 2020

Property, Plant and Equipment [Abstract]

Schedule of Property, Plant and Equipment

	December 31, 2020				December 31, 2019
(Thousands of dollars)	Cost		Net		Cost	Net
Exploration and production ¹	$	19,583,682	8,232,191	2	19,096,323	9,875,727	2
Corporate and other	140,661		36,847		207,066	94,016
Property, plant and equipment	$	19,724,343	8,269,038		19,303,389	9,969,743
¹  Includes unproved mineral rights as follows:	$	649,704	530,194		508,526	121,163

2  Includes $22,940 in 2020 and $24,698 in 2019 related to administrative assets and support equipment.

Schedule of Recognized Impairments

The following table reflects the recognized impairments for the three years ended December 31, 2020.

	December 31,
(Thousands of dollars)	2020		2019	2018
U.S.	$	1,152,515	—	20,000
Other Foreign	39,709		—	—
Corporate	14,060		—	—
	$	1,206,284	—	20,000

Net Changes in Capitalized Exploratory Well Costs

The following table reflects the net changes in capitalized exploratory well costs during the three-year period ended December 31, 2020.

(Thousands of dollars)	2020		2019	2018
Beginning balance at January 1	$	217,326	207,855	155,103
Additions pending the determination of proved reserves	3,999		83,712	59,487
Reclassifications to proved properties based on the determination of proved reserves	—		(61,096)	(2,214)
Capitalized exploration well costs charged to expense	(39,709)		(13,145)	(4,521)
Ending balance at December 31	$	181,616	217,326	207,855

Aging of Capitalized Exploratory Well Costs

The following table provides an aging of capitalized exploratory well costs based on the date the drilling was completed for each individual well and the number of projects for which exploratory well costs has been capitalized.  The projects are aged based on the last well drilled in the project.

	2020				2019				2018
(Thousands of dollars)	Amount		No. of Wells	No. of Projects	Amount		No. of Wells	No. of Projects	Amount		No. of Wells	No. of Projects
Aging of capitalized well costs:
Zero to one year	$	—	—	—	$	63,409	5	5	$	61,096	1	1
One to two years	54,220		5	5	—		—	—	40,523		3	2
Two to three years	—		—	—	27,396		1	—	5,208		1	1
Three years or more	127,396		6	—	126,521		5	—	101,028		4	1
	$	181,616	11	5	$	217,326	11	5	$	207,855	9	5