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, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Assets
Cash	$ 271	$ 387	$ 190	$ 195
Short term investments	1,407	1,660
Other assets	1,078	1,295
Total assets	55,233	55,045
Liabilities
Short term debt	0	350
Long term debt	2,710	2,210
Other liabilities	4,123	4,243
Total liabilities	43,264	43,289
Stockholders' Equity
Common stock ($2.50 par value; 500,000,000 shares authorized; 273,040,243 shares issued; 270,495,998 and 270,274,361 shares outstanding)	683	683	683	683
Additional paid-in capital	2,173	2,153	2,151	2,145
Retained earnings	9,359	9,313	9,645	9,495
Accumulated other comprehensive income (loss)	(173)	(315)	400	442
Treasury stock (2,544,245 and 2,765,882 shares), at cost	(73)	(78)	(84)	(91)
Total stockholders’ equity	11,969	11,756	12,794
Total liabilities and stockholders' equity	$ 55,233	$ 55,045
Balance Sheet Parenthetical
Common stock, par value	$ 2.50	$ 2.50
Common stock, shares authorized	500,000,000	500,000,000
Common stock, shares issued	273,040,243	273,040,243
Common stock, shares outstanding	270,495,998	270,274,361
Treasury stock, shares	2,544,245	2,765,882
Parent Company [Member]
Assets
Investment in subsidiaries	$ 14,202	$ 13,851
Cash	1	4	$ 1	$ 1
Short term investments	487	478
Due from Affiliates	6	0
Other assets	1	1
Total assets	14,697	14,334
Liabilities
Short term debt	0	350
Long term debt	2,680	2,180
Other liabilities	48	48
Total liabilities	2,728	2,578
Stockholders' Equity
Common stock ($2.50 par value; 500,000,000 shares authorized; 273,040,243 shares issued; 270,495,998 and 270,274,361 shares outstanding)	683	683
Additional paid-in capital	2,173	2,153
Retained earnings	9,359	9,313
Accumulated other comprehensive income (loss)	(173)	(315)
Treasury stock (2,544,245 and 2,765,882 shares), at cost	(73)	(78)
Total stockholders’ equity	11,969	11,756
Total liabilities and stockholders' equity	$ 14,697	$ 14,334
Balance Sheet Parenthetical
Common stock, par value	$ 2.50	$ 2.50
Common stock, shares authorized	500,000,000	500,000,000
Common stock, shares issued	273,040,243	273,040,243
Common stock, shares outstanding	270,495,998	270,274,361
Treasury stock, shares	2,544,245	2,765,882