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Share-based compensation - Narrative (Details) - USD ($)
$ in Millions
6 Months Ended 12 Months Ended
Jun. 30, 2016
Dec. 31, 2015
Jul. 15, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Unamortized share compensation expense $ 11.7 $ 13.1  
Unamortized share compensation expense, period of recognition 1 year 5 months 1 day 1 year 4 months  
Omnibus Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Common shares made available     21,627,906
Restricted shares with service and performance condition      
Fair value assumptions      
Remaining contractual life 5 years 7 months 17 days 6 years