Securities (Amortized Cost and Estimated Fair Values of Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Available-for-sale, Amortized Cost
Due in one year or less	$ 4,986
Due after one year through five years	104,294
Due after five years through ten years	212,809
Due after ten years	480,378
Total	802,467
Available-for-sale, Fair Value
Due in one year or less	4,983
Due after one year through five years	103,572
Due after five years through ten years	209,663
Due after ten years	470,875
Total	789,093
Due in one year or less	751
Held-to-maturity, Amortized Cost
Due in one year or less	751
Due after one year through five years	4,705
Due after five years through ten years	10,110
Due after ten years	78,507
Held-to-maturity Securities	94,073	$ 94,609
Held-to-maturity, Fair Value
Held-to-maturity Securities, Debt Maturities, Remainder of Fiscal Year, Fair Value	751
Due after one year through five years	4,721
Due after five years through ten years	10,192
Due after ten years	79,249
Held-to-maturity Securities, Fair Value	$ 94,913	$ 94,596