Consolidated Statements of Stockholder's Equity Consolidated Statements of Stockholder's Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Retained Earnings	Accumulated Other Comprehensive Loss
Beginning balance (in shares) at Dec. 31, 2017		16,448
Beginning balance at Dec. 31, 2017	$ 575,434	$ 2	$ 1,161	$ 618,254	$ (43,983)
Net earnings	2,600			2,600
Performance share, restricted stock unit, and stock option awards (in shares)		13
Performance share, restricted stock unit, and stock option awards	1,267		1,267
Reclassification of the income tax effects of the Tax Cuts and Jobs Act				12,852	(12,852)
Pension and other postretirement employee benefit plans, net of tax	1,419				1,419
Beginning balance (in shares) at Mar. 31, 2018		16,461
Beginning balance at Mar. 31, 2018	580,720	$ 2	2,428	633,706	(55,416)
Beginning balance (in shares) at Dec. 31, 2017		16,448
Beginning balance at Dec. 31, 2017	575,434	$ 2	1,161	618,254	(43,983)
Net earnings	44,005
Beginning balance (in shares) at Sep. 30, 2018		16,461
Beginning balance at Sep. 30, 2018	628,125	$ 2	5,714	675,111	(52,702)
Beginning balance (in shares) at Mar. 31, 2018		16,461
Beginning balance at Mar. 31, 2018	580,720	$ 2	2,428	633,706	(55,416)
Net earnings	6,961			6,961
Performance share, restricted stock unit, and stock option awards	1,552		1,552
Pension and other postretirement employee benefit plans, net of tax	1,336				1,336
Beginning balance (in shares) at Jun. 30, 2018		16,461
Beginning balance at Jun. 30, 2018	590,569	$ 2	3,980	640,667	(54,080)
Net earnings	34,444			34,444
Performance share, restricted stock unit, and stock option awards	1,734		1,734
Pension and other postretirement employee benefit plans, net of tax	1,378				1,378
Beginning balance (in shares) at Sep. 30, 2018		16,461
Beginning balance at Sep. 30, 2018	628,125	$ 2	5,714	675,111	(52,702)
Beginning balance (in shares) at Dec. 31, 2018		16,482
Beginning balance at Dec. 31, 2018	426,396	$ 2	6,403	487,339	(67,348)
Net earnings	3,837			3,837
Performance share, restricted stock unit, and stock option awards (in shares)		33
Performance share, restricted stock unit, and stock option awards	772		772
Pension and other postretirement employee benefit plans, net of tax	1,239				1,239
Beginning balance (in shares) at Mar. 31, 2019		16,515
Beginning balance at Mar. 31, 2019	432,244	$ 2	7,175	491,176	(66,109)
Beginning balance (in shares) at Dec. 31, 2018		16,482
Beginning balance at Dec. 31, 2018	426,396	$ 2	6,403	487,339	(67,348)
Net earnings	(7,560)
Beginning balance (in shares) at Sep. 30, 2019		16,515
Beginning balance at Sep. 30, 2019	425,145	$ 2	8,853	479,779	(63,489)
Beginning balance (in shares) at Mar. 31, 2019		16,515
Beginning balance at Mar. 31, 2019	432,244	$ 2	7,175	491,176	(66,109)
Net earnings	(424)			(424)
Performance share, restricted stock unit, and stock option awards	1,211		1,211
Pension and other postretirement employee benefit plans, net of tax	1,334				1,334
Beginning balance (in shares) at Jun. 30, 2019		16,515
Beginning balance at Jun. 30, 2019	434,365	$ 2	8,386	490,752	(64,775)
Net earnings	(10,973)			(10,973)
Performance share, restricted stock unit, and stock option awards	467		467
Pension and other postretirement employee benefit plans, net of tax	1,286				1,286
Beginning balance (in shares) at Sep. 30, 2019		16,515
Beginning balance at Sep. 30, 2019	$ 425,145	$ 2	$ 8,853	$ 479,779	$ (63,489)