Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Preferred stock, no par value
Preferred stock, authorized	1,000,000	1,000,000
Preferred stock, issued	0	0
Common stock, par value	$ 1	$ 1
Common stock, authorized	1,200,000,000	1,200,000,000
Common stock, issued	351,800,000	350,700,000