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Retirement Plan (Details 3)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Periodic Pension Cost [Line Items]      
Discount rate 1.60% 1.80% 2.00%
Rate of increase in compensation levels 5.00% 5.00% 5.00%
Expected long-term rate of return on pension assets 1.60% 1.80% 2.00%