Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) shares in Millions, $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Statement of Financial Position [Abstract]
Fixed maturities, amortized cost	$ 9,320	$ 8,871
Equity securities, cost	2,970	2,728
Short-term investments, amortized cost	75	0
Land, building and equipment, accumulated depreciation	$ 452	$ 446
Common stock, par value	$ 2	$ 2
Common stock, authorized	500.0	500.0
Common stock, issued	198.3	198.3
Common stock, outstanding	198.3	198.3
Treasury stock, shares	34.5	34.6