Securities - Amortized Cost and Estimated Fair Value of Held-to-Maturity Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Amortized Cost
Due in one year or less	$ 5,980	$ 7,452
Due after one year through five years	51,936	27,480
Due after five years through ten years	81,300	107,187
Due after ten years	0	0
Total held-to-maturity debt securities	139,216	142,119
Fair Value
Due in one year or less	5,979	7,469
Due after one year through five years	52,227	27,866
Due after five years through ten years	82,109	107,497
Due after ten years	0	0
Total held-to-maturity debt securities	$ 140,315	$ 142,832