Reinsurance (Reinsurance Recoverable on Ceded Reinsurance) (Details) - USD ($) $ in Millions		Dec. 31, 2015		Dec. 31, 2014		Dec. 31, 2013	Dec. 31, 2012
Reinsurance Disclosures [Abstract]
Reinsurance recoverable on unpaid losses and loss expenses		$ 10,741	[1]	$ 11,307	[1]	$ 10,612	$ 11,399
Reinsurance recoverable on paid losses and loss expenses	[1]	645		685
Net reinsurance recoverable on losses and loss expenses		$ 11,386		$ 11,992

[1] (1) Net of a provision for uncollectible reinsurance.