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Goodwill and intangible assets (Schedule of Amortization Expense) (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Goodwill and Intangible Assets Disclosure [Abstract]      
Amortization of intangible assets $ 24,413 $ 20,346 $ 14,530
Estimated useful life, intangible assets 14 years    
Estimated future amortization expense      
2014 26,515    
2015 26,455    
2016 25,772    
2017 25,426    
2018 $ 25,336