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, 2018	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets
Cash	$ 310		$ 355	$ 271	$ 387
Short term investments	1,286		1,436
Other assets	1,208		921
Total assets	57,152		56,567
Liabilities
Short term debt	0		150
Long term debt	2,680		2,708
Other liabilities	3,089		3,281
Total liabilities	45,935		44,323
Stockholders' Equity
Common stock ($2.50 par value; 500,000,000 shares authorized; 273,040,243 shares issued; 271,456,978 and 271,205,390 shares outstanding)	683		683
Additional paid-in capital	2,192		2,175
Retained earnings	9,277	$ 9,348	9,414
Accumulated other comprehensive (loss) income	(878)		32
Treasury stock (1,583,265 and 1,834,853 shares), at cost	(57)		(60)
Total stockholders’ equity	11,217		12,244	11,969
Total liabilities and stockholders' equity	$ 57,152		$ 56,567
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)	271,456,978		271,205,390
Treasury stock, shares (in shares)	1,583,265		1,834,853
Parent Company
Assets
Investment in subsidiaries	$ 13,427		$ 14,481
Cash	1		1	$ 1	$ 4
Short term investments	519		638
Amounts due from affiliates	2		4
Other assets	0		1
Total assets	13,949		15,125
Liabilities
Short term debt	0		150
Long term debt	2,680		2,678
Other liabilities	52		53
Total liabilities	2,732		2,881
Stockholders' Equity
Common stock ($2.50 par value; 500,000,000 shares authorized; 273,040,243 shares issued; 271,456,978 and 271,205,390 shares outstanding)	683		683
Additional paid-in capital	2,192		2,175
Retained earnings	9,277		9,414
Accumulated other comprehensive (loss) income	(878)		32
Treasury stock (1,583,265 and 1,834,853 shares), at cost	(57)		(60)
Total stockholders’ equity	11,217		12,244
Total liabilities and stockholders' equity	$ 13,949		$ 15,125