CONSOLIDATED BALANCE SHEETS (UNAUDITED) (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Available for sale securities, amortized cost	$ 197,135	$ 208,911
Held-to-maturity securities, fair value	$ 276,866	$ 233,554
Common stock, par value	$ 1.00	$ 1.00
Common stock, shares authorized	10,000,000	10,000,000
Common stock, shares, outstanding	4,104,317	4,082,188