Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) shares in Millions, $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Fixed maturities, amortized cost	$ 10,653	$ 10,314
Available-for-sale Equity Securities, Amortized Cost Basis	3,294	3,094
Land, building and equipment, accumulated depreciation	$ 259	$ 253
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	35.6	34.4