Debt, Financing Arrangements, and Leases (Tables)

12 Months Ended

Dec. 31, 2017

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



	December 31,
	2017			2016
	(Thousands of Dollars)
5.95% senior unsecured notes due 2017	$	—		$	185,000
6.05% senior unsecured notes due 2018	250,000			250,000
7.125% unsecured debentures due 2025	85,000			85,000
4.0% unsecured debentures due 2027	250,000			—
Debt issuance costs	(2,415		)	(624		)
Unamortized debt discount	(963		)	(216		)
Total long-term debt, including current portion	$	581,622		$	519,160
Long-term debt due within one year	249,874			184,924
Total long-term debt less current portion	$	331,748		$	334,236

Maturities of long-term debt

As of December 31, 2017, cumulative maturities of outstanding long-term debt (at face value) for the next five years are as follows:



	(Thousands of Dollars)
2018: 6.05% senior unsecured notes	$	250,000
Total	$	250,000

Future minimum annual rental 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)
2018	$	2,761
2019	2,787
Total	$	5,548