5. Income Taxes (Tables)
 12 Months Ended
Dec. 31, 2015
Income Tax Disclosure [Abstract]  
Income Tax Expense
    Year ended December 31,  
    2015     2014     2013  
Current  —  Federal   $ 11,848     $ 12,626     $ 12,541  
       —  State     1,528       1,585       1,065  
      13,376       14,211       13,606  
                         
Deferred  —  Federal     (1,364 )      31       (1,063 )
         — State     (67 )     (29 )     140  
      (1,431 )      2       (923 )
Total income tax expense   $ 11,945     $ 14,213     $ 12,683  
Deferred Income Tax Assets and Liabilities with Temporary Differences and Carryforwards
    2015     2014  
Deferred tax assets:            
Benefit plans   $ 1,958     $ 1,535  
Inventories     473       483  
Other     733       158  
Total deferred tax assets   $ 3,164     $ 2,176  
Deferred tax liabilities:                
Property, plant and equipment   $ 9,585     $ 9,648  
Patents and goodwill     2,897       2,926  
Other     91       158  
Total deferred tax liabilities   $ 12,573     $ 12,732  
                 
Net deferred tax liability   $ 9,409     $ 10,556  
                 
Balance Sheet classification:                
Non-current deferred income tax liability   $ 9,989     $ 11,129  
Current deferred income tax asset     580       573  
Net deferred tax liability   $ 9,409     $ 10,556  
Illustration of Total Income Tax Expense Difference with Amount Provided by Applying Statutory Federal Income Tax Rate to Pretax Earnings
    Year ended December 31,  
    2015     2014     2013  
Income tax expense at the statutory federal income tax rate   $ 14,304     $ 14,707     $ 13,743  
Increase (decrease) resulting from:                        
State income taxes     882       934       770  
Section 199 manufacturing deduction     (1,383 )     (1,290 )     (1,307 )
       R&D tax credits     (2,254 )     (393 )     (626 )
Other, net     396       255       103  
Total income tax expense   $ 11,945     $ 14,213     $ 12,683  
Reconciliation of Beginning and Ending Balances of Total Amounts of Gross Unrecognized Tax Benefits
Gross unrecognized tax benefits at January 1, 2013   $ 541  
Increase in tax positions for prior years     11  
Increase in tax positions for current year     0  
Lapse in statutes of limitation     (206 ) 
Gross unrecognized tax benefits at December 31, 2013   $ 346  
Increase in tax positions for prior years     6  
Increase in tax positions for current year     0  
Lapse in statutes of limitation     (223 )
Gross unrecognized tax benefits at December 31, 2014   $ 129  
Increase in tax positions for prior years     122  
Increase in tax positions for current year     0  
Lapse in statutes of limitation     (131 )
Gross unrecognized tax benefits at December 31, 2015   $ 120