Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Investment securities held-to-maturity, fair value	$ 100,483	$ 0
Common Stock, Par Value	$ 1.00	$ 1.00
Common Stock, Shares Authorized	10,000,000	10,000,000
Common Stock, Shares, Issued	4,242,216	4,233,297
Common Stock, Shares, Outstanding	4,242,216	4,233,297