Property, Plant and Equipment (Tables)

9 Months Ended

Sep. 30, 2019

Property, Plant and Equipment [Abstract]

Net Changes in Capitalized Exploratory Well Costs

The following table reflects the net changes in capitalized exploratory well costs during the nine-month periods ended September 30, 2019 and 2018.

(Thousands of dollars)	2019			2018
Beginning balance at January 1	$	207,855		155,103
Additions pending the determination of proved reserves	86,025			41,560
Reclassifications to proved properties based on the determination of proved reserves	—			(2,214	)
Capitalized exploratory well costs charged to expense	(13,145		)	(4,521	)
Balance at September 30	$	280,735		189,928

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 have been capitalized.  The projects are aged based on the last well drilled in the project.

	September 30,
	2019				2018
(Thousands of dollars)	Amount		No. of Wells	No. of Projects	Amount		No. of Wells	No. of Projects
Aging of capitalized well costs:
Zero to one year	$	64,711	5	5	$	46,813	1	1
One to two years	63,615		1	1	41,051		3	2
Two to three years	27,500		1	—	5,208		1	1
Three years or more	124,909		5	—	96,856		4	1
	$	280,735	12	6	$	189,928	9	5