Debt, Financing Arrangements, and Leases (Tables)

12 Months Ended

Dec. 31, 2018

Debt Disclosure [Abstract]

Long-term debt

Long-term debt, presented net of unamortized discount and unamortized debt issuance costs, consists of the following:

	December 31,
	2018			2017
	(Thousands of Dollars)
6.05% senior unsecured notes due 2018	$	—		$	250,000
7.125% unsecured debentures due 2025	85,000			85,000
4.0% unsecured debentures due 2027	500,000			250,000
Debt issuance costs	(4,507		)	(2,415		)
Unamortized debt discount	(4,325		)	(963		)
Total long-term debt, including current portion	576,168			581,622
Long-term debt due within one year	—			249,874
Total long-term debt less current portion	$	576,168		$	331,748

Future Minimum lease payments

Following are the estimated future minimum annual rental payments required under operating leases, which have initial or remaining noncancelable lease terms in excess of one year:

	(Thousands of Dollars)
2019	$	1,942
2020	1,968
2021	1,920
2022	1,909
2023	1,832
Thereafter	11,508
Total	$	21,079