Investment Securities - Summary of Amortized Cost and Fair Value by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 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	8,140
Available-for-sale: Due after five years through ten years	22,640
Available-for-sale: Due after ten years	270,755
Amortized Cost	362,946
Fair Value
Available-for-sale: Due in one year or less	0
Available-for-sale: Due after one year through five years	8,150
Available-for-sale: Due after five years through ten years	22,805
Available-for-sale: Due after ten years	261,169
Fair Value	352,606
Amortized Cost
Held-to-maturity: Due in one year or less	768
Held-to-maturity: Due after one year through five years	4,821
Held-to-maturity: Due after five years through ten years	13,347
Held-to-maturity: Due after ten years	15,963
Amortized Cost	64,893	$ 81,052
Fair Value
Held-to-maturity: Due in one year or less	767
Held-to-maturity: Due after one year through five years	4,763
Held-to-maturity: Due after five years through ten years	13,316
Held-to-maturity: Due after ten years	15,563
Fair Value	63,581	$ 80,920
Collateralized Mortgage Backed Securities
Amortized Cost
Amortized Cost	61,411
Fair Value
Fair Value	60,482
Amortized Cost
Amortized Cost	29,994
Fair Value
Fair Value	$ 29,172