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, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Assets
Cash	$ 419	$ 242
Other assets	1,628	1,473
Total assets	64,026	60,612
Liabilities
Long term debt	2,776	2,679
Other liabilities	3,377	3,325
Total liabilities	51,319	48,397
Stockholders' Equity
Common stock ($2.50 par value; 500,000,000 shares authorized; 273,040,243 shares issued; 271,391,603 and 271,412,591 shares outstanding)	683	683
Additional paid-in capital	2,211	2,203
Retained earnings	9,081	9,348
Accumulated other comprehensive income	803	51
Treasury stock (1,648,640 and 1,627,652 shares), at cost	(71)	(70)
Total stockholders’ equity	12,707	12,215	$ 11,217
Total liabilities and stockholders' equity	$ 64,026	$ 60,612
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)	271,391,603	271,412,591
Treasury stock, shares (in shares)	1,648,640	1,627,652
Parent Company
Assets
Investment in subsidiaries	$ 14,996	$ 14,412
Cash	0	1	$ 1	$ 1
Short term investments	522	521
Amounts due from affiliates	4	2
Other assets	1	1
Total assets	15,523	14,937
Liabilities
Long term debt	2,776	2,679
Other liabilities	40	43
Total liabilities	2,816	2,722
Stockholders' Equity
Common stock ($2.50 par value; 500,000,000 shares authorized; 273,040,243 shares issued; 271,391,603 and 271,412,591 shares outstanding)	683	683
Additional paid-in capital	2,211	2,203
Retained earnings	9,081	9,348
Accumulated other comprehensive income	803	51
Treasury stock (1,648,640 and 1,627,652 shares), at cost	(71)	(70)
Total stockholders’ equity	12,707	12,215
Total liabilities and stockholders' equity	$ 15,523	$ 14,937