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Employee benefit plans - Additional Information (Details) - USD ($)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Disclosure of defined benefit plans [line items]    
Refundable tax of percentage of gain on defined benefit pension plans 50.00%  
Post-employment benefit expense, defined contribution plans $ 404,000 $ 193,000
Employer contributions 718,000 0
Actual return on plan assets 3,439,000 (685,000)
Plan assets    
Disclosure of defined benefit plans [line items]    
Contributions by employer 0 0
Benefit payments $ 26,771,000 $ 4,182,000