Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Securities held for sale, at amortized cost, fair value	$ 11,002	$ 11,461
Common stock, par value	$ 1.25	$ 1.25
Common stock, shares authorized	20,000,000	20,000,000
Common stock, shares issued	7,100,749	7,112,853
Common stock, shares outstanding	7,100,749	7,112,853
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0