Assets and Liabilities Measured at Fair Value (Tables)

12 Months Ended

Dec. 31, 2018

Assets and Liabilities Measured at Fair Value [Abstract]

Carrying Value of Assets and Liabilities Recorded at Fair Value on Recurring Basis

	December 31, 2018					December 31, 2017
(Thousands of dollars)	Level 1		Level 2	Level 3	Total	Level 1	Level 2	Level 3	Total
Assets:
Commodity derivative contracts	$	–	3,837	–	3,837	–	–	–	–
	$	–	3,837	–	3,837	–	–	–	–
Liabilities:
Nonqualified employee         savings plans	$	13,845	–	–	13,845	16,158	–	–	16,158
Contingent consideration		–	–	47,730	47,730	–	–	–	–
Commodity derivative contracts		–	–	–	–	–	39,093	–	39,093
	$	13,845	–	47,730	61,575	16,158	39,093	–	55,251

Carrying Amounts and Estimated Fair Values of Financial Instruments

	December 31,
	2018			2017
(Thousands of dollars)	Carrying Amount		Fair Value	Carrying Amount	Fair Value
Financial assets (liabilities):
Current and long-term debt	$	(3,237,717)	(3,003,388)	(2,916,422)	(2,993,003)

Nonrecurring Fair Value Measurements

	Year Ended December 31,
						Total
					Net Book	Pretax
					Value	(Noncash)
	Fair Value				Prior to	Impairment
(Thousands of dollars)		Level 1	Level 2	Level 3	Impairment	Expense
2018
Assets:
Impaired proved properties
United States Midland	$	–	–	37,690	57,690	20,000
2016
Assets:
Impaired proved properties
Western Canada	$	–	–	71,967	167,055	95,088