Reinsurance (Reinsurance Recoverable on Ceded Reinsurance) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reinsurance Disclosures [Abstract]
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	$ 12,708	$ 10,741	$ 11,307	$ 10,612
Reinsurance recoverable on paid losses and loss expenses	869	645
Net reinsurance recoverable on losses and loss expenses	$ 13,577	$ 11,386