Consolidated Statements of Income (Unaudited) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Income Statement [Abstract]
Revenues		$ 199,897	$ 149,681
Costs and expenses:
Cost of goods sold		139,155	112,498
Selling, general and administrative expenses		13,656	11,368
Deltic merger-related costs	[1]	19,255
Total costs and expenses		172,066	123,866
Operating income		27,831	25,815
Interest expense, net		(5,660)	(4,970)
Non-operating pension and other postretirement benefit costs		(1,857)	(1,906)
Income before income taxes		20,314	18,939
Income tax		(5,717)	(2,018)
Net income		$ 14,597	$ 16,921
Net income per share:
Basic (in dollars per share)		$ 0.29	$ 0.41
Diluted (in dollars per share)		0.29	0.41
Dividends per share (in dollars per share)		$ 0.40	$ 0.375
Weighted-average shares outstanding (in thousands):
Basic (in shares)		50,424,910	40,777,901
Diluted (in shares)		50,786,184	41,070,609

[1] For integration and restructuring costs related to the merger with Deltic see Note 14: Merger, Integration and Other Costs.