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, 2018	$ 1,314,779	$ 67,570	$ 1,659,031	$ (282,391)	$ (129,431)
Balance, beginning of period (shares) at Dec. 31, 2018		67,570
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	6,560			6,560
Shares issued for stock compensation		$ 297	(297)
Shares issued for stock compensation (shares)		297
Equity-based compensation expense	1,617		1,617
Pension plans and OPEB obligations	1,166				1,166
Cash flow hedges	(8,513)				(8,513)
Common dividends, $0.40 per share	(27,065)			(27,065)
Repurchase of common stock	(10,158)	$ (279)		(9,879)
Repurchase of common stock (shares)		(279)
Other transactions, net			99	(99)
Balance, end of period at Mar. 31, 2019	1,278,386	$ 67,588	1,660,450	(312,874)	(136,778)
Balance, end of period (shares) at Mar. 31, 2019		67,588
Balance, beginning of period at Dec. 31, 2018	1,314,779	$ 67,570	1,659,031	(282,391)	(129,431)
Balance, beginning of period (shares) at Dec. 31, 2018		67,570
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	44,262
Cash flow hedges	(25,908)
Repurchase of common stock	$ (25,200)
Repurchase of common stock (shares)	(686,240)
Balance, end of period at Sep. 30, 2019	$ 1,235,729	$ 67,221	1,664,333	(343,747)	(152,078)
Balance, end of period (shares) at Sep. 30, 2019		67,221
Balance, beginning of period at Mar. 31, 2019	1,278,386	$ 67,588	1,660,450	(312,874)	(136,778)
Balance, beginning of period (shares) at Mar. 31, 2019		67,588
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	17,137			17,137
Shares issued for stock compensation		$ 5	(5)
Shares issued for stock compensation (shares)		5
Equity-based compensation expense	1,832		1,832
Pension plans and OPEB obligations	1,008				1,008
Cash flow hedges	(10,417)				(10,417)
Common dividends, $0.40 per share	(26,881)			(26,881)
Repurchase of common stock	(15,015)	$ (407)		(14,608)
Repurchase of common stock (shares)		(407)
Other transactions, net			104	(104)
Balance, end of period at Jun. 30, 2019	1,246,050	$ 67,186	1,662,381	(337,330)	(146,187)
Balance, end of period (shares) at Jun. 30, 2019		67,186
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	20,565			20,565
Shares issued for stock compensation		$ 35	(35)
Shares issued for stock compensation (shares)		35
Equity-based compensation expense	1,913		1,913
Pension plans and OPEB obligations	1,087				1,087
Cash flow hedges	(6,978)				(6,978)
Common dividends, $0.40 per share	$ (26,888)			(26,888)
Repurchase of common stock (shares)	0
Other transactions, net	$ (20)		74	(94)
Balance, end of period at Sep. 30, 2019	1,235,729	$ 67,221	1,664,333	(343,747)	(152,078)
Balance, end of period (shares) at Sep. 30, 2019		67,221
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	67,221
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
Equity-based compensation expense	1,885		1,885
Pension plans and OPEB obligations	15,578				15,578
Cash flow hedges	(38,525)				(38,525)
Common dividends, $0.40 per share	(26,941)			(26,941)
Repurchase of common stock	(12,355)	$ (401)		(11,954)
Repurchase of common stock (shares)		(401)
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
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	67,221
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	$ 66,813
Cash flow hedges	(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	66,872
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
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
Equity-based compensation expense	1,980		1,980
Pension plans and OPEB obligations	2,727				2,727
Cash flow hedges	(1,847)				(1,847)
Common dividends, $0.40 per share	(26,744)			(26,744)
Repurchase of common stock	(3,009)	$ (89)		(2,920)
Repurchase of common stock (shares)		(89)
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
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
Equity-based compensation expense	2,063		2,063
Pension plans and OPEB obligations	2,948				2,948
Cash flow hedges	11,332				11,332
Common dividends, $0.40 per share	$ (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	66,872