Quarterly Results of Operations (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 4,143	$ 3,643	$ 3,606	$ 3,204	$ 3,785	$ 3,628	$ 3,467	$ 3,420	$ 14,596	$ 14,300	$ 12,876
Total benefits and expenses	3,974	3,358	3,411	3,300	3,477	3,281	3,207	3,203	14,043	13,168	12,030
Income before income taxes	169	285	195	(96)	308	347	260	217	553	1,132	846
Net Income (Loss) Attributable to Parent	$ 132	$ 213	$ 158	$ (88)	$ 235	$ 263	$ 202	$ 170	$ 415	$ 870	$ 716
Basic earnings per share:
Earnings Per Share, Basic (in dollars per share)	$ 1.95	$ 3.13	$ 2.49	$ (1.41)	$ 3.75	$ 4.19	$ 3.23	$ 2.70	$ 6.35	$ 13.88	$ 11.25
Earnings Per Share, Diluted (in dollars per share)	$ 1.94	$ 3.12	$ 2.48	$ (1.41)	$ 3.68	$ 4.12	$ 3.18	$ 2.65	$ 6.31	$ 13.62	$ 11.00