Condensed Unaudited Quarterly Financial Data (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
Condensed Income Statements, Captions [Line Items]
Net premiums earned	$ 7,935	$ 8,327	$ 7,891	$ 7,137	$ 7,465	$ 7,908	$ 7,664	$ 7,027	$ 31,290	$ 30,064	$ 29,034
Net Investment Income	858	873	859	836	848	823	828	806	3,426	3,305	3,125
Net Realized Gains Losses	(55)	(155)	(223)	(97)	(687)	19	18	(2)	(530)	(652)	84
Total revenues	8,738	9,045	8,527	7,876	7,626	8,750	8,510	7,831	34,186	32,717	32,243
Losses and loss expenses	4,865	5,052	4,715	4,098	4,610	4,868	4,487	4,102	18,730	18,067	18,454
Policy benefits	225	158	161	196	162	127	150	151	740	590	676
Net income	$ 1,173	$ 1,091	$ 1,150	$ 1,040	$ 355	$ 1,231	$ 1,294	$ 1,082	$ 4,454	$ 3,962	$ 3,861
Basic earnings per share	$ 2.59	$ 2.40	$ 2.52	$ 2.27	$ 0.77	$ 2.66	$ 2.78	$ 2.32	$ 9.77	$ 8.55	$ 8.26
Diluted earnings per share	$ 2.57	$ 2.38	$ 2.50	$ 2.25	$ 0.76	$ 2.64	$ 2.76	$ 2.30	$ 9.71	$ 8.49	$ 8.19