Annual report pursuant to Section 13 and 15(d)

Accrued Liabilities (Reconciliation of Product Warranties) (Details)

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Accrued Liabilities (Reconciliation of Product Warranties) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Accrued Liabilities, Current [Abstract]      
Balance at beginning of period $ 11,455 $ 11,487 $ 11,148
Provision for warranty claims 4,538 3,802 4,560
Warranty claims (4,210) (3,726) (3,708)
Other 556 (108) (513)
Balance at end of period $ 12,339 $ 11,455 $ 11,487