Long-Term Debt (Tables)

12 Months Ended

Dec. 31, 2015

Debt Disclosure [Abstract]

Schedule of long-term debt

	December 31, 2015						December 31, 2014
	Principal Value		Carrying Value (a)		Fair Value (b)		Principal Value		Carrying Value (a)		Fair Value (b)
	(Thousands)
7.76% notes, due 2015 through 2016	$	—	$	—	$	—	$	10,700	$	10,700	$	10,700
5.00% notes, due October 1, 2015	—		—		—		150,000		149,865		154,692
5.15% notes, due March 1, 2018	200,000		199,156		203,490		200,000		198,766		214,626
6.50% notes, due April 1, 2018	500,000		498,360		520,175		500,000		497,630		559,900
8.13% notes, due June 1, 2019	700,000		697,295		760,837		700,000		696,483		843,864
4.88% notes, due November 15, 2021	750,000		742,270		728,063		750,000		740,945		812,558
4.00% EQM notes, due August 1, 2024	500,000		493,401		414,125		500,000		492,633		495,685
7.75% debentures, due July 15, 2026	115,000		109,738		119,372		115,000		109,240		147,036
Medium-term notes:
7.3% to 7.6% Series B, due 2015 through 2023	10,000		9,991		10,241		20,000		19,985		22,279
7.6% Series C, due 2018	8,000		7,983		8,366		8,000		7,975		8,950
8.7% to 9.0% Series A, due 2020 through 2021	35,200		35,149		38,598		35,200		35,131		43,912
	2,818,200		2,793,343		2,803,267		2,988,900		2,959,353		3,314,202
Less debt payable within one year	—		—		—		166,011		165,874		171,081
Total long-term debt	$	2,818,200	$	2,793,343	$	2,803,267	$	2,822,889	$	2,793,479	$	3,143,121

(a)     Carrying value represents principal value less unamortized debt issuance costs and debt discounts.

(b)    Fair value is measured using Level 2 inputs.