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employee future benefits - Defined contribution pension plans (Details)
12 Months Ended
Dec. 31, 2018
Defined contribution pension plans  
Number of defined contribution plans 3
Matching contribution to employees' 5% contribution 100.00%
Employer matching contribution over 5% of employee's contribution 80.00%
Minimum  
Defined contribution pension plans  
Employees contribution to the plan (as a per cent) 3.00%
Maximum  
Defined contribution pension plans  
Employees contribution to the plan (as a per cent) 6.00%
Employees' contribution 100% matched (as a per cent) 5.00%