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, 2019	Jan. 01, 2019	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets
Cash	$ 242		$ 310
Short term investments	1,861		1,286
Other assets	1,473	$ 1,445	1,208
Total assets	60,612		57,152
Liabilities
Long term debt	2,679		2,680
Other liabilities	3,325	$ 3,328	3,089
Total liabilities	48,397		45,935
Stockholders' Equity
Common stock ($2.50 par value; 500,000,000 shares authorized; 273,040,243 shares issued; 271,412,591 and 271,456,978 shares outstanding)	683		683
Additional paid-in capital	2,203		2,192
Retained earnings	9,348		9,277	$ 9,348
Accumulated other comprehensive income (loss)	51		(878)
Treasury stock (1,627,652 and 1,583,265 shares), at cost	(70)		(57)
Total stockholders’ equity	12,215		11,217		$ 12,244
Total liabilities and stockholders' equity	$ 60,612		$ 57,152
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)	271,412,591		271,456,978
Treasury stock, shares (in shares)	1,627,652		1,583,265
Parent Company
Assets
Investment in subsidiaries	$ 14,412		$ 13,427
Cash	1		1		$ 1	$ 1
Short term investments	521		519
Amounts due from affiliates	2		2
Other assets	1		0
Total assets	14,937		13,949
Liabilities
Long term debt	2,679		2,680
Other liabilities	43		52
Total liabilities	2,722		2,732
Stockholders' Equity
Common stock ($2.50 par value; 500,000,000 shares authorized; 273,040,243 shares issued; 271,412,591 and 271,456,978 shares outstanding)	683		683
Additional paid-in capital	2,203		2,192
Retained earnings	9,348		9,277
Accumulated other comprehensive income (loss)	51		(878)
Treasury stock (1,627,652 and 1,583,265 shares), at cost	(70)		(57)
Total stockholders’ equity	12,215		11,217
Total liabilities and stockholders' equity	$ 14,937		$ 13,949