Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Loss [Member]
Balance, beginning of period at Dec. 31, 2019	$ 1,226,831	$ 67,221	$ 1,666,299	$ (359,330)	$ (147,359)
Balance, beginning of period (shares) at Dec. 31, 2019		67,221,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	(16,832)			(16,832)
Shares issued for stock compensation		$ 131	(131)
Shares issued for stock compensation (shares)		131,000
Equity-based compensation expense	1,885		1,885
Pension plans and OPEB obligations, net of tax	15,578				15,578
Cash flow hedges, net of tax	(38,525)				(38,525)
Dividends on common stock	(26,941)			(26,941)
Repurchase of common stock	(12,355)	$ (401)		(11,954)
Repurchase of common stock (shares)		(401,000)
Other transactions, net	(27)		69	(96)
Balance, end of period at Mar. 31, 2020	1,149,614	$ 66,951	1,668,122	(415,153)	(170,306)
Balance, end of period (shares) at Mar. 31, 2020		66,951,000
Balance, beginning of period at Dec. 31, 2019	1,226,831	$ 67,221	1,666,299	(359,330)	(147,359)
Balance, beginning of period (shares) at Dec. 31, 2019		67,221,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	66,813
Cash flow hedges, net of tax	(29,040)
Repurchase of common stock	$ (15,400)
Repurchase of common stock (shares)	(489,850)
Balance, end of period at Sep. 30, 2020	$ 1,196,077	$ 66,872	1,672,351	(388,000)	(155,146)
Balance, end of period (shares) at Sep. 30, 2020		66,872,000
Balance, beginning of period at Mar. 31, 2020	1,149,614	$ 66,951	1,668,122	(415,153)	(170,306)
Balance, beginning of period (shares) at Mar. 31, 2020		66,951,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	2,638			2,638
Shares issued for stock compensation		$ 9	(9)
Shares issued for stock compensation (shares)		9,000
Equity-based compensation expense	1,980		1,980
Pension plans and OPEB obligations, net of tax	2,727				2,727
Cash flow hedges, net of tax	(1,847)				(1,847)
Dividends on common stock	(26,744)			(26,744)
Repurchase of common stock	(3,009)	$ (89)		(2,920)
Repurchase of common stock (shares)		(89,000)
Other transactions, net	117		91	26
Balance, end of period at Jun. 30, 2020	1,125,476	$ 66,871	1,670,184	(442,153)	(169,426)
Balance, end of period (shares) at Jun. 30, 2020		66,871,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	81,007			81,007
Shares issued for stock compensation		$ 1	(1)
Shares issued for stock compensation (shares)		1,000
Equity-based compensation expense	2,063		2,063
Pension plans and OPEB obligations, net of tax	2,948				2,948
Cash flow hedges, net of tax	11,332				11,332
Dividends on common stock	$ (26,749)			(26,749)
Repurchase of common stock (shares)	0
Other transactions, net			105	(105)
Balance, end of period at Sep. 30, 2020	$ 1,196,077	$ 66,872	1,672,351	(388,000)	(155,146)
Balance, end of period (shares) at Sep. 30, 2020		66,872,000
Balance, beginning of period at Dec. 31, 2020	$ 1,304,953	$ 66,876	1,674,576	(315,510)	(120,989)
Balance, beginning of period (shares) at Dec. 31, 2020	66,876,000	66,876,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	$ 131,106			131,106
Shares issued for stock compensation		$ 166	(166)
Shares issued for stock compensation (shares)		166,000
Equity-based compensation expense	1,930		1,930
Pension plans and OPEB obligations, net of tax	2,953				2,953
Cash flow hedges, net of tax	64,107				64,107
Dividends on common stock	(27,484)			(27,484)
Other transactions, net	(16)		81	(97)
Balance, end of period at Mar. 31, 2021	1,477,549	$ 67,042	1,676,421	(211,985)	(53,929)
Balance, end of period (shares) at Mar. 31, 2021		67,042,000
Balance, beginning of period at Dec. 31, 2020	$ 1,304,953	$ 66,876	1,674,576	(315,510)	(120,989)
Balance, beginning of period (shares) at Dec. 31, 2020	66,876,000	66,876,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	$ 384,678
Cash flow hedges, net of tax	$ 39,580
Repurchase of common stock (shares)	0
Balance, end of period at Sep. 30, 2021	$ 1,660,106	$ 67,100	1,679,332	(13,561)	(72,765)
Balance, end of period (shares) at Sep. 30, 2021	67,100,000	67,100,000
Balance, beginning of period at Mar. 31, 2021	$ 1,477,549	$ 67,042	1,676,421	(211,985)	(53,929)
Balance, beginning of period (shares) at Mar. 31, 2021		67,042,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	187,905			187,905
Shares issued for stock compensation		$ 3	(3)
Shares issued for stock compensation (shares)		3,000
Equity-based compensation expense	2,140		2,140
Pension plans and OPEB obligations, net of tax	2,845				2,845
Cash flow hedges, net of tax	(31,163)				(31,163)
Dividends on common stock	(27,489)			(27,489)
Other transactions, net	2		103	(101)
Balance, end of period at Jun. 30, 2021	1,611,789	$ 67,045	1,678,661	(51,670)	(82,247)
Balance, end of period (shares) at Jun. 30, 2021		67,045,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	65,667			65,667
Shares issued for stock compensation		$ 55	(55)
Shares issued for stock compensation (shares)		55,000
Equity-based compensation expense	2,275		2,275
Pension plans and OPEB obligations, net of tax	2,846				2,846
Cash flow hedges, net of tax	6,636				6,636
Dividends on common stock	(27,489)			(27,489)
Other transactions, net	(1,618)		(1,549)	(69)
Balance, end of period at Sep. 30, 2021	$ 1,660,106	$ 67,100	$ 1,679,332	$ (13,561)	$ (72,765)
Balance, end of period (shares) at Sep. 30, 2021	67,100,000	67,100,000