Other Operating Revenues and Expenses (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Other operating revenues:
Gain on sales of assets	$ 11,685	$ 4,467	$ 1,496
Marketing revenue	3,708	2,021	581
Total other operating revenues	15,393	6,488	2,077
Other operating expenses:
Loss on sales of assets	(2,511)	(1,233)	(523)
Marketing expense	0	(658)	0
Impairment of inventory	(36)	(210)	(510)
Total other operating expenses	$ (2,547)	$ (2,101)	$ (1,033)