Consolidated Condensed Statements of Income (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Revenues	$ 606,950		$ 570,289		$ 525,134
Costs and expenses:
Cost of goods sold1	429,789	[1]	408,772	[1]	390,773	[1]
Selling, general and administrative expenses	44,655		50,397		49,419
Environmental remediation charges	0		3,522		0
Total costs and expenses	474,444		462,691		440,192
Operating income	132,506		107,598		84,942
Interest expense, net	(22,909)		(23,132)		(25,539)
Income before income taxes	109,597		84,466		59,403
Income taxes	(19,687)		(13,885)		(16,809)
Net income	89,910		70,581		42,594
Net income per share:
Basic (in dollars per share)	$ 2.21		$ 1.74		$ 1.06
Diluted (in dollars per share)	$ 2.20		$ 1.73		$ 1.05
Distributions per share (in dollars per share)	$ 1.425		$ 1.28		$ 1.24
Asset impairment charge					$ 107

[1] A 2012 asset impairment charge of $107 was reclassified to cost of goods sold.