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Fair value measurements (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
ASSETS:
Cash equivalents	$ 119,194	$ 137,712
Trust assets	5,998	8,067
Surety bonds	1,870	1,987
TOTAL	127,062	147,766
Fair Value, Inputs, Level 1 [Member]
ASSETS:
Cash equivalents	119,194	137,712
Trust assets	5,998	8,067
Surety bonds	1,870	1,987
TOTAL	127,062	147,766
Fair Value, Inputs, Level 2 [Member]
ASSETS:
Cash equivalents	0	0
Trust assets	0	0
Surety bonds	0	0
TOTAL	0	0
Fair Value, Inputs, Level 3 [Member]
ASSETS:
Cash equivalents	0	0
Trust assets	0	0
Surety bonds	0	0
TOTAL	$ 0	$ 0

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- Definition

Fair values as of the balance sheet date of all surety bonds for workers compensation insurance as of the balance sheet date.

+ References

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- Definition

Fair value as of the balance sheet date of all trust assets as of the balance sheet date.

+ References

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X

- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef



-Publisher FASB



-Name Accounting Standards Codification



-Topic 820



-SubTopic 10



-Section 50



-Paragraph 2



-Subparagraph (a)



-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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- Definition

Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef



-Publisher FASB



-Name Accounting Standards Codification



-Glossary Cash



-URI http://asc.fasb.org/extlink&oid=6506951



Reference 2: http://www.xbrl.org/2003/role/presentationRef



-Publisher FASB



-Name Accounting Standards Codification



-Glossary Cash Equivalents



-URI http://asc.fasb.org/extlink&oid=6507016

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