Long-Term Debt (Tables)

12 Months Ended

Dec. 31, 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.

December 31,	2013					2012
	Carrying Amount			Market Value		Carrying Amount			Market Value
(Thousands of dollars)
Debentures:
Notes, 4.45%, due 2020	$	125,000		$	130,953	$	125,000		$	141,771
Notes, 6.1%, due 2041		125,000			141,873		125,000			165,779
Notes, 3.875%, due 2022		250,000			252,485		250,000			277,950
Notes, 4.875%, due 2043		250,000			257,280		—			—
8% Series, due 2026		75,000			96,263		75,000			111,501
Medium-term notes, 7.59% series, due 2017		25,000			28,741		25,000			30,710
Medium-term notes, 7.78% series, due 2022		25,000			30,586		25,000			34,637
Medium-term notes, 7.92% series, due 2027		25,000			31,497		25,000			36,953
Medium-term notes, 6.76% series, due 2027		7,500			8,468		7,500			10,058
Unamortized discount		(5,560	)				(3,403	)
		901,940					654,097
Revolving credit facility and commercial paper		10,000			10,000		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			20,150		20,000			20,571
5.80% 2003 Series E, due 2038		—			—		15,000			15,102
5.25% 2004 Series A, due 2034		65,000			64,522		65,000			66,955
5.00% 2004 Series B, due 2033		31,200			30,284		31,200			31,655
4.85% 2005 Series A, due 2035		100,000			95,192		100,000			101,184
4.75% 2006 Series A, due 2036		24,855			22,974		24,855			25,189
Unamortized discount		(2,776	)				(3,195	)
		438,279					491,130
NPL credit facility		—			—		41,562			41,562
NPL other debt obligations		42,213			42,119		20,721			20,991
		1,392,432					1,318,510
Less: current maturities		(11,105	)				(50,137	)
Long-term debt, less current maturities	$	1,381,327				$	1,268,373

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, 2013			December 31, 2012
2003 Series A		1.43	%		1.71	%
2008 Series A		1.41	%		1.59	%
2009 Series A		1.01	%		1.14	%
Tax-exempt Series A		1.07	%		1.25	%

Estimated Maturities of Long-Term Debt

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

2014	$	11,105
2015		11,100
2016		10,200
2017		42,210
2018		2,598