Assets and Liabilities Measured at Fair Value (Tables)

12 Months Ended

Dec. 31, 2017

Assets and Liabilities Measured at Fair Value [Abstract]

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

	December 31, 2017					December 31, 2016
(Thousands of dollars)	Level 1		Level 2	Level 3	Total	Level 1	Level 2	Level 3	Total
Liabilities:
Nonqualified employee         savings plans	$	16,158	–	–	16,158	13,904	–	–	13,904
Commodity derivative contracts		–	39,093	–	39,093	–	48,864	–	48,864
Foreign currency exchange         derivative contracts		–	–	–	–	–	73	–	73
	$	16,158	39,093	–	55,251	13,904	48,937	–	62,841

Carrying Amounts and Estimated Fair Values of Financial Instruments

	At December 31,
	2017			2016
(Thousands of dollars)	Carrying Amount		Fair Value	Carrying Amount	Fair Value
Financial assets (liabilities):
Canadian government securities with maturities             greater than 90 days at the date of acquisition	$	-	-	111,542	111,331
Current and long-term debt		(2,916,422)	(2,993,003)	(2,992,567)	(2,951,992)

Nonrecurring Fair Value Measurements

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