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Related party transactions (Details) (Sterling Group Parters I, L.P.)	0 Months Ended
	Aug. 20, 2012
Sterling Group Parters I, L.P.
Related Party Transaction [Line Items]
Common shares distributed by Sterling to its limited and general partners	4,626,265

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- Definition

Related Party Transactions, Common Stock Distributed by Related Party to Affiliated Partners

+ References

No definition available.

+ Details

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Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

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- Details

Name:	us-gaap_RelatedPartyTransactionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration