Consolidated Statements of Stockholders' Equity - 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, 2016	$ 156,274	$ 40,519	$ 355,274	$ (128,775)	$ (110,744)
Balance, beginning of period (shares) at Dec. 31, 2016		40,519
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	86,453			86,453
Equity-based compensation expense	$ 4,722		4,722
Shares issued for stock compensation		$ 93	(93)
Shares issued for stock compensation (share)		93
Repurchase of common stock (shares)	0
Pension plans and OPEB obligations	$ 15,889				15,889
Cash flow hedges	4				4
Common distributions, $1.525 per share 2017, $1.60 per share 2018, $1.60 per share 2019	(61,931)			(61,931)
Other transactions, net	(869)		(759)	(110)
Balance, end of period at Dec. 31, 2017	200,542	$ 40,612	359,144	(104,363)	(94,851)
Balance, end of period (shares) at Dec. 31, 2017		40,612
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	122,880			122,880
Equity-based compensation expense	$ 8,206		8,206
Shares issued for stock compensation		$ 162	(162)
Shares issued for stock compensation (share)		162
Repurchase of common stock (shares)	0
Pension plans and OPEB obligations	$ (8,900)				(8,900)
Cash flow hedges	(2,415)				(2,415)
Cumulative effects of adoption of accounting standards	1,299			24,564	(23,265)
Common distributions, $1.525 per share 2017, $1.60 per share 2018, $1.60 per share 2019	(102,333)			(102,333)
Common stock issued for Deltic merger	1,142,775	$ 21,981	1,120,794
Common stock issued for Deltic merger (shares)		21,981
Deltic earnings and profit special distribution, $3.54 per share	(44,435)	$ 4,815	172,750	(222,000)
Deltic earnings and profit special distribution (shares)		4,815
Other transactions, net	(2,840)		(1,701)	(1,139)
Balance, end of period at Dec. 31, 2018	1,314,779	$ 67,570	1,659,031	(282,391)	(129,431)
Balance, end of period (shares) at Dec. 31, 2018		67,570
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	55,661			55,661
Equity-based compensation expense	7,272		7,272
Shares issued for stock compensation		$ 337	(337)
Shares issued for stock compensation (share)		337
Repurchase of common stock	$ (25,173)	$ (686)		(24,487)
Repurchase of common stock (shares)	(686,240)	(686)
Pension plans and OPEB obligations	$ 512				512
Cash flow hedges	(18,440)				(18,440)
Common distributions, $1.525 per share 2017, $1.60 per share 2018, $1.60 per share 2019	(107,722)			(107,722)
Other transactions, net	(58)		333	(391)
Balance, end of period at Dec. 31, 2019	$ 1,226,831	$ 67,221	$ 1,666,299	$ (359,330)	$ (147,359)
Balance, end of period (shares) at Dec. 31, 2019		67,221