Derivative Financial Instruments (Summary of Collateral Positions with Counterparties) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Derivative [Line Items]
Cash collateral received from counterparties	$ 66	$ 44
Cash collateral posted to counterparties	277	386
Dealer Counterparties [Member]
Derivative [Line Items]
Cash collateral received from counterparties	60	44
Derivatives in a net gain position secured by that collateral	70	46
Unsecured positions in a net gain after collateral postings	10	3
Cash collateral posted to counterparties	267	356
Derivatives in a net loss position secured by that collateral	269	357
Additional collateral required for decline in credit ratings below investment grade	4	4
Central Clearing Parties [Member]
Derivative [Line Items]
Cash collateral received from counterparties	7	0
Derivatives in a net gain position secured by that collateral	6	26
Cash collateral posted to counterparties	12	43
Derivatives in a net loss position secured by that collateral	38	43
Securities pledged to central clearing parties	$ 141	$ 82