Condensed Unaudited Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Income Statements, Captions [Line Items]
Net premiums earned	$ 7,465	$ 7,908	$ 7,664	$ 7,027	$ 7,218	$ 7,807	$ 7,237	$ 6,772	$ 30,064	$ 29,034	$ 28,749
Net Investment Income	848	823	828	806	797	813	770	745	3,305	3,125	2,865
Net Realized Gain (Loss)	(687)	19	18	(2)					(652)
Realized Gain (Loss) on Marketable Securities, Cost Method Investments, and Other Investments						(10)	101	(7)		84	(145)
Total revenues	7,626	8,750	8,510	7,831	8,015	8,610	8,108	7,510	32,717	32,243	31,469
Losses and loss expenses	4,610	4,868	4,487	4,102	4,272	6,247	4,146	3,789	18,067	18,454	16,052
Policy benefits	162	127	150	151	176	169	163	168	590	676	588
Net income	$ 355	$ 1,231	$ 1,294	$ 1,082	$ 1,533	$ (70)	$ 1,305	$ 1,093	$ 3,962	$ 3,861	$ 4,135
Basic earnings per share	$ 0.77	$ 2.66	$ 2.78	$ 2.32	$ 3.29	$ (0.15)	$ 2.79	$ 2.33	$ 8.55	$ 8.26	$ 8.94
Diluted earnings per share	$ 0.76	$ 2.64	$ 2.76	$ 2.30	$ 3.27	$ (0.15)	$ 2.77	$ 2.31	$ 8.49	$ 8.19	$ 8.87