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Commitments and Contingencies - Changes in the Liability for Product Warranty Claim Costs (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Changes in the liability for product warranty claim costs      
Balance at beginning of period $ 339 $ 134 $ 35
Accruals for warranties issued during the period 787 312 102
Settlements (in cash or in kind) during the period (850) (107) (3)
Balance at end of period $ 276 $ 339 $ 134