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Valuation and Qualifying Accounts Valuation and Qualifying Accounts (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Employee severance accruals      
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]      
Balance at beginning of year $ 0 $ 1,517 $ 280
Charged to costs and expenses 1,649 571 3,641
Deduction 508 2,088 2,404
Balance at end of year 1,141 0 1,517
Valuation allowance of the deferred tax asset      
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]      
Balance at beginning of year 244 209 189
Charged to costs and expenses 32 35 20
Deduction 0 0 0
Balance at end of year 276 244 209
SEC Schedule, 12-09, Valuation Allowance, Current [Member]      
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]      
Balance at beginning of year 835 746 729
Charged to costs and expenses 1,111 1,248 1,155
Deduction 1,177 1,159 1,138
Balance at end of year 769 835 746
SEC Schedule, 12-09, Valuation Allowance, Noncurrent [Member]      
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]      
Balance at beginning of year 2,278 2,166 2,042
Charged to costs and expenses 730 950 943
Deduction 857 838 819
Balance at end of year $ 2,151 $ 2,278 $ 2,166