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Commitments and contingencies (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Commitments and Contingencies Disclosure [Abstract]      
Rent expense $ 19.0 $ 16.1 $ 11.0
Amount of letters of credit $ 16.6