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Other Comprehensive Income (Loss), Summary of Unrealized Gains and Losses on Postretirement Benefits (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]                      
Balance $ 184,661 $ 183,304 $ 181,372 $ 181,321 $ 179,882 $ 177,537 $ 174,910 $ 172,521 $ 181,321 $ 172,521 $ 167,498
Reclassification adjustment for effect of enacted tax law changes                   0  
Other comprehensive loss 3,419 (1,677) (904) (2,933) (1,349) (263) 174 60 (2,095) (1,378) (858)
Balance 191,542 $ 184,661 $ 183,304 181,372 181,321 $ 179,882 $ 177,537 174,910 191,542 181,321 172,521
Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss)                      
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]                      
Balance       $ (147)       $ (102) (147) (102) (156)
Unrealized gains (losses) during the period                 233 (30) 84
Related deferred taxes                 (49) 11 (30)
Reclassification adjustment for effect of enacted tax law changes                 0 (26) 0
Other comprehensive loss                 184 (45) 54
Balance $ 37       $ (147)       $ 37 $ (147) $ (102)