Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares |
Sep. 30, 2017 |
Dec. 31, 2016 |
|---|---|---|
| Statement Of Financial Position [Abstract] | ||
| Common stock, par value | $ 1 | $ 1 |
| Common stock, shares authorized | 60,000,000 | 60,000,000 |
| Common stock, shares issued | 26,063,641 | 25,894,782 |
| Treasury stock, shares | 3,542,333 | 3,470,084 |