Reinsurance (Consolidated Reinsurance) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Premiums written [Abstract]
Direct									$ 31,543	$ 19,879	$ 20,069
Assumed									3,440	3,932	3,321
Ceded									(6,838)	(6,098)	(5,591)
Net									28,145	17,713	17,799
Premiums earned [Abstract]
Direct									31,811	19,355	19,555
Assumed									3,744	3,676	3,336
Ceded									(6,806)	(5,818)	(5,465)
Net premiums earned	$ 7,059	$ 7,688	$ 7,405	$ 6,597	$ 4,207	$ 4,719	$ 4,360	$ 3,927	28,749	17,213	17,426
Reinsurance recoveries on losses and loss expenses incurred									$ 4,100	$ 3,100	$ 3,100