Property, Plant and Equipment (Tables)

3 Months Ended

Mar. 31, 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		623	39,653
Reclassifications to proved properties based on the determination of proved reserves		–	(13,370)
Capitalized exploratory well costs charged to expense		–	(8,360)
Balance at March 31	$	176,263	166,423

Aging of Capitalized Exploratory Well Costs

		March 31,
	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	$	13,642	2	1	$	48,202	2	2
One to two years		27,757	1	1		53,729	3	3
Two to three years		49,642	2	2		–	–	–
Three years or more		85,222	7	1		64,492	6	–
	$	176,263	12	5	$	166,423	11	5