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Goodwill and Other Intangible Assets - Summary of Intangible Assets (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Goodwill and Intangible Assets Disclosure [Abstract]    
Gross carrying amount at beginning of year $ 22,114 $ 22,114
Additions 0 0
Accumulated amortization (18,437) (17,204)
Gross carrying amount at end of year $ 3,677 $ 4,910