Long-term Debt (Tables)

12 Months Ended

Dec. 31, 2017

Financing Arrangements and Long-Term Debt [Abstract]

Schedule of Long-term Debt

	December 31,
(Thousands of dollars)	2017		2016
Notes payable
3.50% notes, due December 2017	$	-	550,000
4.00% notes, due June 2022		500,000	500,000
4.45% notes, due December 20221		600,000	600,000
6.875% notes, due August 2024		550,000	550,000
5.75% notes, due August 2025		550,000	-
7.05% notes, due May 2029		250,000	250,000
5.875% notes, due December 20421		350,000	350,000
Total notes payable		2,800,000	2,800,000
Unamortized debt issuance cost and discount on notes payable		(27,433)	(23,835)
Total notes payable, net of unamortized discount		2,772,567	2,776,165
Capitalized lease obligation, due through March 2029		143,855	216,402
Total debt including current maturities		2,916,422	2,992,567
Current maturities		(9,902)	(569,817)
Total long-term debt	$	2,906,520	2,422,750

1Due to a series of ratings changes by credit agencies, the paying interest rates on the notes due December 2022 and December 2042 decreased from 4.7% to 4.45% and 6.125% to 5.875%, respectively, at December 2017.