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, 2017	$ 200,542	$ 40,612	$ 359,144	$ (104,363)	$ (94,851)
Balance, beginning of period (shares) at Dec. 31, 2017		40,612
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	14,597			14,597
Shares issued for stock compensation		$ 162	(162)
Shares issued for stock compensation (shares)		162
Common stock issued for Deltic merger	1,142,775	$ 21,981	1,120,794
Common stock issued for Deltic merger (shares)		21,981
Equity-based compensation expense	3,279		3,279
Pension plans and OPEB obligations	1,725				1,725
Cash flow hedges	(990)				(990)
Cumulative effects of adoption of accounting standards	1,299			24,564	(23,265)
Common dividends, $0.40 per share	(25,102)			(25,102)
Other transactions, net	(2,683)		(2,653)	(30)
Balance, end of period at Mar. 31, 2018	1,335,442	$ 62,755	1,480,402	(90,334)	(117,381)
Balance, end of period (shares) at Mar. 31, 2018		62,755
Balance, beginning of period at Dec. 31, 2017	200,542	$ 40,612	359,144	(104,363)	(94,851)
Balance, beginning of period (shares) at Dec. 31, 2017		40,612
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	60,745
Cash flow hedges	$ 259
Repurchase of common stock (shares)	0
Balance, end of period at Jun. 30, 2018	$ 1,360,926	$ 62,754	1,482,048	(69,426)	(114,450)
Balance, end of period (shares) at Jun. 30, 2018		62,754
Balance, beginning of period at Mar. 31, 2018	1,335,442	$ 62,755	1,480,402	(90,334)	(117,381)
Balance, beginning of period (shares) at Mar. 31, 2018		62,755
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	46,148			46,148
Shares issued for stock compensation		$ (1)	1
Shares issued for stock compensation (shares)		(1)
Equity-based compensation expense	1,610		1,610
Pension plans and OPEB obligations	1,682				1,682
Cash flow hedges	1,249				1,249
Common dividends, $0.40 per share	$ (25,101)			(25,101)
Repurchase of common stock (shares)	0
Other transactions, net	$ (104)		35	(139)
Balance, end of period at Jun. 30, 2018	1,360,926	$ 62,754	1,482,048	(69,426)	(114,450)
Balance, end of period (shares) at Jun. 30, 2018		62,754
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	67,570,000	67,570
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 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	67,570,000	67,570
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 23,697
Cash flow hedges	(18,930)
Repurchase of common stock	$ (25,200)
Repurchase of common stock (shares)	(700,000)
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	67,186,000	67,186
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	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)	(400,000)	(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	67,186,000	67,186