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Investment Securities - Narrative (Details)
$ in Thousands
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
USD ($)
security
Sep. 30, 2017
USD ($)
Sep. 30, 2018
USD ($)
security
Sep. 30, 2017
USD ($)
Dec. 31, 2017
USD ($)
security
Schedule of Available-for-sale Securities and Held-to-maturity Securities [Line Items]          
Securities transferred from available-for-sale to held-to-maturity | $     $ 27,422 $ 128,965 $ 129,000
Unrealized pre-tax loss from transfer of available-for-sale securities to held-to-maturity | $     278   3,000
Amortization of net unrealized losses on securities transferred from available-for-sale to held-to-maturity | $ $ 128 $ 135 $ 396 $ 299 $ 299
Number of investment securities in unrealized loss position | security 253   253   198
Number of investment securities in unrealized loss position longer than 12 months | security 83   83    
Number of investment securities in unrealized loss position less than 12 months | security 170   170