Long-Term Debt (Tables)

9 Months Ended

Sep. 30, 2013

Debt Disclosure [Abstract]

Schedule of Estimated Fair Values of Long-Term Debt

The external credit rating, coupon rate, and maturity of each security are considered in the valuation, as applicable.

	September 30, 2013					December 31, 2012
	Carrying Amount			Market Value		Carrying Amount			Market Value
(Thousands of dollars)
Debentures:
Notes, 4.45%, due 2020	$	125,000		$	133,784	$	125,000		$	141,771
Notes, 6.1%, due 2041		125,000			142,243		125,000			165,779
Notes, 3.875%, due 2022		250,000			256,543		250,000			277,950
8% Series, due 2026		75,000			98,735		75,000			111,501
Medium-term notes, 7.59% series, due 2017		25,000			29,106		25,000			30,710
Medium-term notes, 7.78% series, due 2022		25,000			31,356		25,000			34,637
Medium-term notes, 7.92% series, due 2027		25,000			32,302		25,000			36,953
Medium-term notes, 6.76% series, due 2027		7,500			8,680		7,500			10,058
Unamortized discount		(3,259	)				(3,403	)
		654,241					654,097
Revolving credit facility and commercial paper		150,000			150,000		111,000			111,000
Industrial development revenue bonds:
Variable-rate bonds:
Tax-exempt Series A, due 2028		50,000			50,000		50,000			50,000
2003 Series A, due 2038		50,000			50,000		50,000			50,000
2008 Series A, due 2038		50,000			50,000		50,000			50,000
2009 Series A, due 2039		50,000			50,000		50,000			50,000
Fixed-rate bonds:
5.55% 1999 Series D, due 2038		—			—		8,270			8,375
5.45% 2003 Series C, due 2038		—			—		30,000			30,152
5.25% 2003 Series D, due 2038		20,000			20,095		20,000			20,571
5.80% 2003 Series E, due 2038		—			—		15,000			15,102
5.25% 2004 Series A, due 2034		65,000			61,914		65,000			66,955
5.00% 2004 Series B, due 2033		31,200			29,886		31,200			31,655
4.85% 2005 Series A, due 2035		100,000			94,475		100,000			101,184
4.75% 2006 Series A, due 2036		24,855			23,280		24,855			25,189
Unamortized discount		(2,808	)				(3,195	)
		438,247					491,130
NPL credit facility		4,000			4,000		41,562			41,562
NPL other debt obligations		44,956			44,977		20,721			20,991
		1,291,444					1,318,510
Less: current maturities		(11,052	)				(50,137	)
Long-term debt, less current maturities	$	1,280,392				$	1,268,373