Derivative Financial Instruments (Summary of Collateral Positions with Counterparties) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Derivative [Line Items]
Cash collateral to secure loss from counterparty related to derivative credit risk	$ 68	$ 41
Cash Collateral Posted Related To Derivative Credit Risk	549	607
Dealer Counterparties [Member]
Derivative [Line Items]
Cash collateral to secure loss from counterparty related to derivative credit risk	69	44
Derivatives In Gain Position For Which Collateral Has Been Received	73	42
Cash Collateral Posted Related To Derivative Credit Risk	525	603
Derivatives In Loss Position For Which Collateral Has Been Posted	525	610
Additional Collateral Required For Decline in Credit Ratings Below Investment Grade	2	10
Unsecured position with derivative dealers	4	0
Central Clearing Parties [Member]
Derivative [Line Items]
Cash Collateral Posted Related To Derivative Credit Risk	26	111
Derivatives In Loss Position For Which Collateral Has Been Posted	26	7
Securities Pledged As Initial Margin To Central Clearing Parties	43	0
Significant Unsecured Positions In Gain With Central Clearing Parties	$ 228	$ 0