Derivative Financial Instruments (Summary of Collateral Positions with Counterparties) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Derivative [Line Items]
Cash collateral received from counterparties	$ 190	$ 44
Cash collateral, including initial margin, posted to counterparties	342	386
Dealer Counterparties [Member]
Derivative [Line Items]
Cash collateral received from counterparties	191	44
Derivatives in a net gain position secured by that collateral	201	46
Unsecured positions in a net gain after collateral postings	10	3
Cash collateral, including initial margin, posted to counterparties	227	356
Derivatives in a net loss position secured by that collateral	231	357
Additional collateral required for decline in credit ratings below investment grade	3	4
Central Clearing Parties [Member]
Derivative [Line Items]
Derivatives in a net gain position secured by that collateral	0	26
Cash collateral, including initial margin, posted to counterparties	114	43
Derivatives in a net loss position secured by that collateral	129	43
Securities pledged to central clearing parties	$ 116	$ 82