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.

No definition available.

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

Commitments and Contingencies (Details 2) (USD $) In Thousands, unless otherwise specified							6 Months Ended	12 Months Ended
	Jun. 30, 2013	Nov. 30, 2011	Mar. 31, 2013 Q3C	Nov. 30, 2012 Q3C	Jun. 30, 2013 Letters of credit	Dec. 31, 2012 Letters of credit	Jun. 30, 2013 Bonding	Dec. 31, 2012 Bonding
Commitments and contingencies
Total commercial letters of credit outstanding					$ 9,023	$ 6,168
Bid and completion bonds issued and outstanding							1,388,279	1,298,589
Withdrawal liability recorded	$ 7,500	$ 7,500	$ 119	$ 85