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Commitments and contingencies - Future rental payments under operating leases (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2014
Mar. 31, 2013
Future rental payments under operating leases in respect of equipment rentals and leased premises    
Remainder of 2014 $ 8,213  
2015 10,875  
2016 10,862  
2017 10,939  
2018 10,464  
2019 9,874  
2020 and thereafter 73,640  
Future rental payments under operating leases 134,867  
Rental expenses $ 3,148 $ 2,553