Annual and transition report of foreign private issuers pursuant to Section 13 or 15(d)

Retirement Plans and Employee Rights Upon Termination (Narrative) (Details)

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Retirement Plans and Employee Rights Upon Termination (Narrative) (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Retirement Benefits [Abstract]      
Severance liabilities $ 3,900,000 $ 3,700,000  
Employee contribution percentage 4.00%    
Employer contributions matching amount $ 3,500    
Percent of employee contribution matched by the Company 50.00%    
Employer contribution amount $ 3,700,000 3,800,000 $ 4,200,000
Future benefits to employees $ 3,000,000 $ 2,700,000