Senior Notes (Tables)

12 Months Ended

Dec. 31, 2017

Debt Disclosure [Abstract]

Schedule of long-term debt

	December 31, 2017						December 31, 2016
	Principal Value		Carrying Value (a)		Fair Value (b)		Principal Value		Carrying Value (a)		Fair Value (b)
	(Thousands)
5.15% Notes, due March 1, 2018	$	—	$	—	$	—	$	200,000	$	199,545	$	207,180
6.50% Notes, due April 1, 2018	—		—		—		500,000		499,089		527,205
8.13% Notes, due June 1, 2019	700,000		698,918		755,153		700,000		698,106		789,271
Floating Rate Notes due October 1, 2020	500,000		497,206		501,325		—		—		—
2.50% Notes due October 1, 2020	500,000		497,169		497,670		—		—		—
4.88% Notes, due November 15, 2021	750,000		744,920		801,953		750,000		743,595		801,218
3.00% Notes due October 1, 2022	750,000		742,364		743,550		—		—		—
4.00% EQM Notes, due August 1, 2024	500,000		494,939		504,110		500,000		494,170		493,125
7.75% debentures, due July 15, 2026	115,000		110,732		135,024		115,000		110,235		141,800
4.125% EQM Notes, due December 1, 2026	500,000		492,413		501,990		500,000		491,562		488,460
3.90% Notes due October 1, 2027	1,250,000		1,238,707		1,245,200		—		—		—
Medium-term notes:
7.42% Series B, due 2023	10,000		10,000		11,433		10,000		9,998		11,677
7.6% Series C, due 2018	8,000		7,999		8,012		8,000		7,991		8,375
8.7% to 9.0% Series A, due 2020 through 2021	35,200		35,187		40,510		35,200		35,168		41,906
	5,618,200		5,570,554		5,745,930		3,318,200		3,289,459		3,510,217
Less Senior Notes payable within one year	8,000		7,999		8,012		—		—		—
Total Senior Notes	$	5,610,200	$	5,562,555	$	5,737,918	$	3,318,200	$	3,289,459	$	3,510,217

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

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