Reinsurance (Consolidated Reinsurance) (Details) - USD ($) $ 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
Premiums written [Abstract]
Direct									$ 36,848	$ 34,782	$ 33,137
Assumed									3,276	3,186	3,239
Ceded									(7,849)	(7,389)	(7,132)
Net									32,275	30,579	29,244
Premiums earned [Abstract]
Direct									35,876	34,108	32,782
Assumed									3,107	3,175	3,332
Ceded									(7,693)	(7,219)	(7,080)
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
Reinsurance recoveries on losses and loss expenses incurred									$ 4,900	$ 5,600	$ 5,500