Financial Instruments and Risk Management (Tables)

3 Months Ended

Mar. 31, 2017

Financial Instruments and Risk Management [Abstract]

Fair Value of Derivative Instruments Not Designated as Hedging Instruments

	March 31, 2017			December 31, 2016
(Thousands of dollars)	Asset (Liability) Derivatives			Asset (Liability) Derivatives
Type of Derivative Contract	Balance Sheet Location	Fair Value		Balance Sheet Location	Fair Value
Commodity	Accounts payable	$	(7,753)	Accounts payable	$	(48,864)
Foreign exchange	Accounts receivable		152	Accounts payable		(73)

Recognized Gains and Losses for Derivative Instruments Not Designated as Hedging Instruments

			Gain (Loss)
		Three Months Ended
(Thousands of dollars)		March 31,
Type of Derivative Contract	Statement of Operations Location		2017	2016
Commodity	Sales and other operating revenues	$	37,077	13,189
Foreign exchange	Interest and other income		225	305
		$	37,302	13,494

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

	March 31, 2017					December 31, 2016
(Thousands of dollars)	Level 1		Level 2	Level 3	Total	Level 1	Level 2	Level 3	Total
Assets:
Foreign currency exchange         derivative contracts	$	–	152	–	152	–	–	–	–
	$	–	152	–	152	–	–	–	–
Liabilities:
Nonqualified employee         savings plans	$	14,074	–	–	14,074	13,904	–	–	13,904
Commodity derivative contracts		–	7,753	–	7,753	–	48,864	–	48,864
Foreign currency exchange         derivative contracts		–	–	–	–	–	73	–	73
	$	14,074	7,753	–	21,827	13,904	48,937	–	62,841

Nonrecurring Fair Value Measurements

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