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Goodwill and intangible assets - Additional Information (Details) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Goodwill and Intangible Assets Disclosure [Abstract]        
Amortization expense related to intangible assets $ 1,800,000 $ 1,800,000 $ 3,500,000 $ 3,600,000
Impairments of goodwill or intangible assets $ 0 $ 0 $ 0 $ 0