Schedule II. Condensed Financial Information of Registrant (Parent Company) (Schedule of Condensed Financial Information of Registrant, Balance Sheets) (Details) - USD ($) $ / shares in Units, $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets
Cash	$ 355	$ 271	$ 387	$ 190
Short term investments	1,436	1,407
Other assets	1,133	1,078
Total assets	56,567	55,233
Liabilities
Short term debt	150	0
Long term debt	2,708	2,710
Other liabilities	4,253	4,123
Total liabilities	44,323	43,264
Stockholders' Equity
Common stock ($2.50 par value; 500,000,000 shares authorized; 273,040,243 shares issued; 271,205,390 and 270,495,998 shares outstanding)	683	683
Additional paid-in capital	2,175	2,173
Retained earnings	9,414	9,359
Accumulated other comprehensive income (loss)	32	(173)
Treasury stock (1,834,853 and 2,544,245 shares), at cost	(60)	(73)
Total stockholders’ equity	12,244	11,969	11,756
Total liabilities and stockholders' equity	$ 56,567	$ 55,233
Balance Sheet Parenthetical
Common stock, par value (in usd per share)	$ 2.50	$ 2.50
Common stock, shares authorized (in shares	500,000,000	500,000,000
Common stock, shares issued (in shares)	273,040,243	273,040,243
Common stock, shares outstanding (in shares)	271,205,390	270,495,998
Treasury stock, shares (in shares)	1,834,853	2,544,245
Parent Company
Assets
Investment in subsidiaries	$ 14,481	$ 14,202
Cash	1	1	$ 4	$ 1
Short term investments	638	487
Amounts due from affiliates	4	6
Other assets	1	1
Total assets	15,125	14,697
Liabilities
Short term debt	150	0
Long term debt	2,678	2,680
Other liabilities	53	48
Total liabilities	2,881	2,728
Stockholders' Equity
Common stock ($2.50 par value; 500,000,000 shares authorized; 273,040,243 shares issued; 271,205,390 and 270,495,998 shares outstanding)	683	683
Additional paid-in capital	2,175	2,173
Retained earnings	9,414	9,359
Accumulated other comprehensive income (loss)	32	(173)
Treasury stock (1,834,853 and 2,544,245 shares), at cost	(60)	(73)
Total stockholders’ equity	12,244	11,969
Total liabilities and stockholders' equity	$ 15,125	$ 14,697