Consolidated balance sheets (Parenthetical) - $ / shares	Jun. 30, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Common stock, par value	$ 1	$ 1
Common stock, shares authorized	75,000,000	75,000,000
Common stock, shares issued	30,683,353	30,535,517
Common stock, shares outstanding	30,683,353	30,535,517