Investment Securities - Summary of Amortized Cost and Fair Value by Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 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	8,672
Available-for-sale: Due after five years through ten years	47,548
Available-for-sale: Due after ten years	261,031
Amortized Cost	317,251	$ 312,584
Fair Value
Available-for-sale: Due in one year or less	0
Available-for-sale: Due after one year through five years	8,638
Available-for-sale: Due after five years through ten years	47,394
Available-for-sale: Due after ten years	252,905
Total available-for-sale securities	308,937
Amortized Cost
Held-to-maturity: Due in one year or less	515
Held-to-maturity: Due after one year through five years	5,044
Held-to-maturity: Due after five years through ten years	43,312
Held-to-maturity: Due after ten years	18,906
Held-to-maturity, Amortized Cost	67,777	81,052
Fair Value
Held-to-maturity: Due in one year or less	515
Held-to-maturity: Due after one year through five years	5,006
Held-to-maturity: Due after five years through ten years	42,639
Held-to-maturity: Due after ten years	18,598
Held-to-maturity, Fair Value	$ 66,758	$ 80,920