Investment Securities - Summary of Amortized Cost and Fair Value by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Amortized Cost
Available-for-sale: Due in one year or less	$ 1,136
Available-for-sale: Due after one year through five years	10,315
Available-for-sale: Due after five years through ten years	43,777
Available-for-sale: Due after ten years	268,560
Available-for-sale, Amortized Cost	323,788
Fair Value
Available-for-sale: Due in one year or less	1,139
Available-for-sale: Due after one year through five years	10,513
Available-for-sale: Due after five years through ten years	45,080
Available-for-sale: Due after ten years	267,411
Available-for-sale, Fair Value	324,143
Amortized Cost
Held-to-maturity: Due in one year or less	1,204
Held-to-maturity: Due after one year through five years	5,413
Held-to-maturity: Due after five years through ten years	43,555
Held-to-maturity: Due after ten years	33,567
Held-to-maturity, Amortized Cost	83,739	$ 98,211
Fair Value
Held-to-maturity: Due in one year or less	1,206
Held-to-maturity: Due after one year through five years	5,465
Held-to-maturity: Due after five years through ten years	44,335
Held-to-maturity: Due after ten years	33,633
Held-to-maturity, Fair Value	$ 84,639	$ 98,261