Consolidated Statements of Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Income Statement [Abstract]
Revenues	$ 149,681	$ 127,896
Costs and expenses:
Cost of goods sold	112,783	109,815
Selling, general and administrative expenses	12,989	13,009
Total costs and expenses	125,772	122,824
Operating income	23,909	5,072
Interest expense, net	(4,970)	(6,025)
Income (loss) before income taxes	18,939	(953)
Income tax (provision) benefit	(2,018)	1,110
Net income	$ 16,921	$ 157
Net income per share:
Basic (in dollars per share)	$ 0.41
Diluted (in dollars per share)	0.41
Distributions per share (in dollars per share)	$ 0.375	$ 0.375
Weighted-average shares outstanding (in thousands):
Basic (in shares)	40,777,901	40,875,179
Diluted (in shares)	41,070,609	40,959,675