Condensed Unaudited Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ 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
Condensed Income Statements, Captions [Line Items]
Income tax benefit related to 2017 Tax Act									$ (450)	$ 0	$ 0
Net premiums earned	$ 7,218	$ 7,807	$ 7,237	$ 6,772	$ 7,059	$ 7,688	$ 7,405	$ 6,597	29,034	28,749	17,213
Net Investment Income	797	813	770	745	744	739	708	674	3,125	2,865	2,194
Net realized gains (losses) including OTTI	0	(10)	101	(7)	365	100	(216)	(394)	84	(145)	(420)
Total revenues	8,015	8,610	8,108	7,510	8,168	8,527	7,897	6,877	32,243	31,469	18,987
Losses and loss expenses	4,272	6,247	4,146	3,789	3,855	4,269	4,254	3,674	18,454	16,052	9,484
Policy benefits	176	169	163	168	161	155	146	126	676	588	543
Net income	$ 1,533	$ (70)	$ 1,305	$ 1,093	$ 1,610	$ 1,360	$ 726	$ 439	$ 3,861	$ 4,135	$ 2,834
Basic earnings per share	$ 3.29	$ (0.15)	$ 2.79	$ 2.33	$ 3.44	$ 2.90	$ 1.55	$ 0.98	$ 8.26	$ 8.94	$ 8.71
Diluted earnings per share	$ 3.27	$ (0.15)	$ 2.77	$ 2.31	$ 3.41	$ 2.88	$ 1.54	$ 0.97	$ 8.19	$ 8.87	$ 8.62
Loss from Catastrophes		$ 1,500