Property, Plant and Equipment (Tables)

9 Months Ended

Sep. 30, 2018

Property, Plant And Equipment [Abstract]

Net Changes in Capitalized Exploratory Well Costs

(Thousands of dollars)	2018		2017
Beginning balance at January 1	$	175,640	148,500
Additions pending the determination of proved reserves		41,940	51,614
Reclassifications to proved properties based on the determination of proved reserves		(2,214)	(13,370)
Capitalized exploratory well costs charged to expense		(4,521)	(8,360)
Balance at September 30	$	210,845	178,384

Aging of Capitalized Exploratory Well Costs

		September 30,
	2018				2017
(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	$	46,813	1	1	$	41,609	3	2
One to two years		41,051	3	2		8,430	2	2
Two to three years		5,208	1	1		43,197	1	1
Three years or more		117,773	5	2		85,148	7	1
	$	210,845	10	6	$	178,384	13	6