Consolidated Balance Sheet (Parenthetical) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 1.00	$ 1.00
Preferred stock, shares authorized	20,000,000	20,000,000
Preferred stock, shares issued	575,000	575,000
Preferred stock, liquidation preference per share	$ 1,000	$ 1,000
Cumulative mandatory convertible series	8.00%	8.00%
Common stock, par value	$ 1.00	$ 1.00
Common stock, shares authorized	600,000,000	600,000,000
Common stock, shares issued	315,053,615	316,523,200