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Multiemployer plan (Details 2) - Operating Engineers Local 800 and WY Contractors Association Pension Plan for WY [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2016
Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Additional multiemployer plan withdrawal obligation	$ 2.4
Multiemployer Plans, Pension [Member]
Multiemployer Plans [Line Items]
Multiemployer plans, withdrawal obligation		$ 16.4

X

- Definition

The amount of loss pertaining to the specified contingency that was charged against earnings in the period, including the effects of revisions in previously reported estimates.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	debit
Period Type:	duration

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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Period Type:	duration

X

- Definition

Amount of the obligation recognized by withdrawing from a pension or postretirement benefit plan to which two or more unrelated employers contribute where assets contributed by one participating employer may be used to provide benefits to employees of other participating employers.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Details

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- Details

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Namespace Prefix:
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Balance Type:
Period Type:

X

- Details

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Balance Type:
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