Consolidated Balance Sheets (Parenthetical) - USD ($) shares in Millions, $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Statement of Financial Position [Abstract]
Fixed maturities, amortized cost	$ 9,324	$ 8,871
Equity securities, cost	2,938	2,728
Land, building and equipment, accumulated depreciation	$ 459	$ 446
Common stock, par value	$ 2	$ 2
common stock, authorized	500.0	500.0
Common stock, issued	198.3	198.3
Common stock, shares outstanding	198.3	198.3
Treasury stock, shares	34.4	34.6