Long-Term Debt (Tables)

12 Months Ended

Dec. 31, 2012

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.

December 31,	2012					2011
	Carrying Amount			Market Value		Carrying Amount			Market Value
(Thousands of dollars)
Debentures:
Notes, 7.625%, due 2012	$	—		$	—	$	200,000		$	204,312
Notes, 4.45%, due 2020		125,000			141,771		125,000			128,673
Notes, 6.1%, due 2041		125,000			165,779		125,000			143,074
Notes, 3.875%, due 2022		250,000			277,950		—			—
8% Series, due 2026		75,000			111,501		75,000			96,340
Medium-term notes, 7.59% series, due 2017		25,000			30,710		25,000			30,199
Medium-term notes, 7.78% series, due 2022		25,000			34,637		25,000			31,932
Medium-term notes, 7.92% series, due 2027		25,000			36,953		25,000			31,648
Medium-term notes, 6.76% series, due 2027		7,500			10,058		7,500			8,510
Unamortized discount		(3,403	)				(2,087	)
		654,097					605,413
Revolving credit facility and commercial paper		111,000			111,000		109,000			109,000
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:
6.10% 1999 Series A, due 2038		—			—		12,410			12,410
5.95% 1999 Series C, due 2038		—			—		14,320			14,449
5.55% 1999 Series D, due 2038		8,270			8,375		8,270			8,253
5.45% 2003 Series C, due 2038 (rate resets in March 2013)		30,000			30,152		30,000			31,332
5.25% 2003 Series D, due 2038		20,000			20,571		20,000			19,583
5.80% 2003 Series E, due 2038 (rate resets in March 2013)		15,000			15,102		15,000			15,634
5.25% 2004 Series A, due 2034		65,000			66,955		65,000			64,291
5.00% 2004 Series B, due 2033		31,200			31,655		31,200			30,283
4.85% 2005 Series A, due 2035		100,000			101,184		100,000			94,836
4.75% 2006 Series A, due 2036		24,855			25,189		24,855			23,179
Unamortized discount		(3,195	)				(3,360	)
		491,130					517,695
NPL credit facility		41,562			41,562		16,566			16,566
NPL other debt obligations		20,721			20,991		4,802			4,814
		1,318,510					1,253,476
Less: current maturities		(50,137	)				(322,618	)
Long-term debt, less current maturities	$	1,268,373				$	930,858

Summary of Effective Interest Rates on Variable-Rate IDRBs

The effective interest rates on the variable-rate IDRBs are included in the table below:

	December 31, 2012			December 31, 2011
2003 Series A		1.71	%		0.83	%
2008 Series A		1.59	%		1.62	%
2009 Series A		1.14	%		1.56	%
Tax-exempt Series A		1.25	%		2.22	%

Estimated Maturities of Long-Term Debt

Estimated maturities of long-term debt for the next five years are (in thousands):

2013	$	50,137
2014		5,249
2015		46,705
2016		4,143
2017		137,049