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Summary of significant accounting policies (Allowance) (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Accounting Policies [Abstract]
Allowance—beginning of period	$ 0	$ (10)	$ 0	$ (5)
Provision for bad debts	0	0	(10)	(52)
Write-off, net of recoveries and other	0	10	0	57
Allowance—end of period	$ 0	$ 0	$ (10)	$ 0

Amount of the current period expense charged against operations, the offset which is generally to the allowance for doubtful accounts for the purpose of reducing receivables, including notes receivable, to an amount that approximates their net realizable value (the amount expected to be collected).

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