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	Sep. 30, 2014	Dec. 31, 2013
Inventory, Net [Abstract]
Equipment inventory	$ 22,503	$ 13,657
Commodities	2,433	3,186
Inventory valuation allowance	(911)	(706)
Inventories, net	$ 24,025	$ 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

Sep. 30, 2014

Dec. 31, 2013

Equipment inventory

$ 22,503

$ 13,657

Commodities

2,433

3,186

Inventory valuation allowance

(911)

(706)

Inventories, net

$ 24,025

$ 16,137