Nature of operations and summary of significant accounting policies Nature of Operations and Summary of Significant Accounting Policies--Inventories (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Inventory, Net [Abstract]
Equipment inventory	$ 19,457	$ 13,657
Commodities	2,751	3,186
Inventory valuation allowance	(779)	(706)
Inventories, net	$ 21,429	$ 16,137

Nature of operations and summary of significant accounting policies Nature of Operations and Summary of Significant Accounting Policies--Inventories (Details) (USD $)
In Thousands, unless otherwise specified

Jun. 30, 2014

Dec. 31, 2013

Equipment inventory

$ 19,457

$ 13,657

Commodities

2,751

3,186

Inventory valuation allowance

(779)

(706)

Inventories, net

$ 21,429

$ 16,137