Financing Arrangements and Debt (Tables)

12 Months Ended

Dec. 31, 2020

Debt Disclosure [Abstract]

Schedule of Long-term Debt

	December 31,
(Thousands of dollars)	2020		2019
Notes payable
4.00% notes, due June 2022	$	259,291	260,251
4.95% notes, due December 2022 ¹	317,067		318,417
6.875% notes, due August 2024	542,428		550,000
5.75% notes, due August 2025	548,675		550,000
5.875% notes, due December 2027	543,249		550,000
7.05% notes, due May 2029	250,000		250,000
6.375% notes, due December 2042 ¹	349,000		350,000
Total notes payable	2,809,710		2,828,668
Unamortized debt issuance cost and discount on notes payable	(21,643)		(25,287)
Total notes payable, net of unamortized discount	2,788,067		2,803,381
Senior Unsecured Revolving Credit Facility	200,000		—
Total long-term debt	$	2,988,067	2,803,381

1 Coupon rate may fluctuate 25 basis points if rating is periodically downgraded or upgraded by S&P and Moody’s.