Investment Securities - Summary of Amortized Cost and Fair Value by Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Amortized Cost
Available-for-sale: Due in one year or less	$ 0
Available-for-sale: Due after one year through five years	7,124
Available-for-sale: Due after five years through ten years	43,042
Available-for-sale: Due after ten years	249,145
Available-for-sale, Amortized Cost	299,311
Fair Value
Available-for-sale: Due in one year or less	0
Available-for-sale: Due after one year through five years	7,116
Available-for-sale: Due after five years through ten years	43,443
Available-for-sale: Due after ten years	241,385
Available-for-sale, Fair Value	291,944
Amortized Cost
Held-to-maturity: Due in one year or less	1,392
Held-to-maturity: Due after one year through five years	5,061
Held-to-maturity: Due after five years through ten years	45,316
Held-to-maturity: Due after ten years	21,486
Held-to-maturity, Amortized Cost	73,255	$ 81,052
Fair Value
Held-to-maturity: Due in one year or less	1,389
Held-to-maturity: Due after one year through five years	5,023
Held-to-maturity: Due after five years through ten years	44,731
Held-to-maturity: Due after ten years	21,164
Held-to-maturity, Fair Value	$ 72,307	$ 80,920