Common stock (Employee options vested and exercised) (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
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Grant-date fair value of options vested during period	$ 517	$ 881	$ 469	$ 470
Intrinsic value of options exercised during period	$ 5,923	$ 2,565	$ 261	$ 1

The total accumulated difference between fair values of underlying shares on dates of exercise and exercise price on options which were exercised (or share units converted) into shares during the reporting period under the plan.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (d)(2)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph c(2)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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Balance Type:	debit
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