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Retirement Plans - Safe Harbor plan (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Employees' savings plan      
Matching contribution by the company (as a percent) 100.00%    
Expenses recognized under 401(K) plan $ 10,962 $ 6,659 $ 7,948
Minimum      
Employees' savings plan      
Age of employees to be eligible to participate in the defined contribution plan 18 years    
Percentage of annual base compensation that participants may elect to contribute 1.00%    
Maximum      
Employees' savings plan      
Percentage of annual base compensation that participants may elect to contribute 50.00%    
Percentage of employees salary for which the company contributes a matching contribution 4.00%    
Discretionary matching contribution (as a percent)     2.00%