Quarterly Results of Operations (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 3,785	$ 3,628	$ 3,467	$ 3,420	$ 3,279	$ 3,227	$ 3,196	$ 3,174	$ 14,300	$ 12,876	$ 12,516
Total benefits and expenses	3,477	3,281	3,207	3,203	3,141	2,904	2,949	3,036	13,168	12,030	11,373
Income before income taxes	308	347	260	217	138	323	247	138	1,132	846	1,143
Net Income (Loss) Attributable to Parent	$ 235	$ 263	$ 202	$ 170	$ 110	$ 301	$ 205	$ 100	$ 870	$ 716	$ 1,822
Basic earnings per share:
Earnings Per Share, Basic (in dollars per share)	$ 3.75	$ 4.19	$ 3.23	$ 2.70	$ 1.75	$ 4.76	$ 3.19	$ 1.55	$ 13.88	$ 11.25	$ 28.28
Earnings Per Share, Diluted (in dollars per share)	$ 3.68	$ 4.12	$ 3.18	$ 2.65	$ 1.72	$ 4.68	$ 3.13	$ 1.52	$ 13.62	$ 11.00	$ 27.71