XML 38 R28.htm IDEA: XBRL DOCUMENT v3.8.0.1
Goodwill and intangible assets - Additional Information (Details) - USD ($)
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]    
Amortization expense related to intangible assets $ 1,800,000 $ 1,900,000
Impairments of goodwill or intangible assets $ 0 $ 0