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Employee benefit plans - Additional information (Details) - USD ($)
 1 Months Ended 12 Months Ended
Sep. 30, 2020
Dec. 31, 2020
Dec. 31, 2019
Disclosure of defined benefit plans [line items]      
Refundable tax of percentage of gain on defined benefit pension plans   50.00%  
Gains (losses) arising from settlements, net defined benefit liability (asset) $ 5,000    
Post-employment benefit expense, defined contribution plans   $ 339,000 $ 404,000
Employer contributions   344,000 718,000
Actual return on plan assets   546,000 (3,439,000)
Plan assets      
Disclosure of defined benefit plans [line items]      
Contributions by employer   0 0
Benefit payments   $ 18,224,000 $ 26,771,000