Consolidated Statements of Income - USD ($) shares in Millions, $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues
Premiums										$ 25,683	$ 24,534	$ 23,874
Net investment income										2,397	2,302	2,379
Fee income										447	458	460
Net realized investment gains	[1]									216	68	3
Other revenues										159	263	99
Total revenues		$ 7,451	$ 7,325	$ 7,184	$ 6,942	$ 7,193	$ 6,961	$ 6,785	$ 6,686	28,902	27,625	26,815
Claims and expenses
Claims and claim adjustment expenses										17,467	15,070	13,723
Amortization of deferred acquisition costs										4,166	3,985	3,885
General and administrative expenses										4,170	4,154	4,094
Interest expense										369	363	373
Total claims and expenses		6,591	7,005	6,394	6,182	5,891	6,014	5,898	5,769	26,172	23,572	22,075
Income before income taxes		860	320	790	760	1,302	947	887	917	2,730	4,053	4,740
Income tax expense		309	27	195	143	359	231	223	226	674	1,039	1,301
Net income		$ 551	$ 293	$ 595	$ 617	$ 943	$ 716	$ 664	$ 691	$ 2,056	$ 3,014	$ 3,439
Net income per share
Basic (in dollars per share)		$ 2.00	$ 1.06	$ 2.13	$ 2.19	$ 3.32	$ 2.48	$ 2.27	$ 2.33	$ 7.39	$ 10.39	$ 10.99
Diluted (in dollars per share)		$ 1.98	$ 1.05	$ 2.11	$ 2.17	$ 3.28	$ 2.45	$ 2.24	$ 2.30	$ 7.33	$ 10.28	$ 10.88
Weighted average number of common shares outstanding
Basic (in shares)										276.0	288.1	310.6
Diluted (in shares)										278.6	291.0	313.9
Cash dividends declared per common share (in dollars per share)										$ 2.83	$ 2.62	$ 2.38

[1] Total other-than-temporary impairment (OTTI) losses were $(13) million, $(40) million and $(54) million for the years ended December 31, 2017, 2016 and 2015, respectively. Of total OTTI, credit losses of $(14) million, $(29) million and $(52) million for the years ended December 31, 2017, 2016 and 2015, respectively, were recognized in net realized investment gains. In addition, unrealized gains (losses) from other changes in total OTTI of $1 million, $(11) million and $(2) million for the years ended December 31, 2017, 2016 and 2015, respectively, were recognized in other comprehensive income (loss) as part of changes in net unrealized gains on investment securities having credit losses recognized in the consolidated statements of income.