Reinsurance (Consolidated Reinsurance) (Details) - USD ($) $ 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
Premiums written [Abstract]
Direct									$ 33,137	$ 31,543	$ 19,879
Assumed									3,239	3,440	3,932
Ceded									(7,132)	(6,838)	(6,098)
Net									29,244	28,145	17,713
Premiums earned [Abstract]
Direct									32,782	31,811	19,355
Assumed									3,332	3,744	3,676
Ceded									(7,080)	(6,806)	(5,818)
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
Reinsurance recoveries on losses and loss expenses incurred									$ 5,500	$ 4,100	$ 3,100