Investment Securities - Schedule of Amortized Cost and Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	$ 13,523	$ 905
Amortized cost, Due in one to five years	18,420	28,332
Amortized cost, Due in five to ten years	19,463	19,218
Amortized cost, Due in over ten years	89,417	67,016
Amortized cost, Total	140,823	115,471
Total debt securities, Amortized cost	628,910	541,028
Fair value, Due in one year or less	13,683	925
Fair value, Due in one to five years	18,316	28,878
Fair value, Due in five to ten years	19,209	19,588
Fair value, Due in over ten years	87,579	68,098
Fair value, Total	138,787	117,489
Available-for-sale debt securities, at fair value	606,501	536,270
Mortgage-backed Securities - Residential
Schedule Of Available For Sale Securities [Line Items]
Total debt securities, Amortized cost	488,087	425,557
Available-for-sale debt securities, at fair value	$ 467,714	$ 418,781