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Revenue Recognition Policies (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2018
Jul. 01, 2017
Jun. 30, 2018
Jul. 01, 2017
Dec. 31, 2017
Dec. 31, 2016
Distributor sales accruals [Line Items]            
Beginning Balance $ 32,706 $ 31,952 $ 36,680 $ 34,479 $ 34,479 $ 32,487
Sales allowances 25,365 23,250 49,553 44,770 89,009 86,896
Credits issued (19,348) (20,051) (47,798) (43,522) (87,403) (85,341)
Foreign currency (691) 582 (403) 6 595 437
Ending Balance $ 38,032 $ 35,733 $ 38,032 $ 35,733 $ 36,680 $ 34,479