Reinsurance (Consolidated Reinsurance) (Details) - USD ($) $ 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
Premiums written [Abstract]
Direct									$ 34,782	$ 33,137	$ 31,543
Assumed									3,186	3,239	3,440
Ceded									(7,389)	(7,132)	(6,838)
Net									30,579	29,244	28,145
Premiums earned [Abstract]
Direct									34,108	32,782	31,811
Assumed									3,175	3,332	3,744
Ceded									(7,219)	(7,080)	(6,806)
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
Reinsurance recoveries on losses and loss expenses incurred									$ 5,600	$ 5,500	$ 4,100