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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	(16,832)			(16,832)
Dividends on common stock	(26,941)			(26,941)
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)
Dividends on common stock	(26,941)			(26,941)
Repurchase of common stock	$ (12,355)	$ (401)		(11,954)
Repurchase of common stock (shares)	(400,917)	(401)
Other transactions, net	$ (27)		69	(96)
Balance, March 31, 2021 at Mar. 31, 2020	1,149,614	$ 66,951	1,668,122	(415,153)	(170,306)
Balance, March 31, 2021 at Mar. 31, 2020		66,951
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	(16,832)			(16,832)
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	$ 131,106			131,106
Dividends on common stock	(27,484)			(27,484)
Shares issued for stock compensation		$ 166	(166)
Shares issued for stock compensation (shares)		166
Equity-based compensation expense	1,930		1,930
Pension plans and OPEB obligations	2,953				2,953
Cash flow hedges	64,107				64,107
Dividends on common stock	$ (27,484)			(27,484)
Repurchase of common stock (shares)	0
Other transactions, net	$ (16)		81	(97)
Balance, March 31, 2021 at Mar. 31, 2021	$ 1,477,549	$ 67,042	$ 1,676,421	(211,985)	$ (53,929)
Balance, March 31, 2021 at Mar. 31, 2021	67,042,000	67,042
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	$ 131,106			$ 131,106