Derivatives (Estimated Fair Value of Derivative Assets and Liabilities after Master Netting Agreements and Cash Collateral) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 1,600	$ 2,370
Derivative Liability, Fair Value, Gross Liability	811	970
Netting [Member]
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset	1,642	2,436
Derivative Liability, Fair Value, Gross Liability	835	1,016
Amounts offset in the consoilidated balance sheets, Assets	0	0
Amounts offset in the consoilidated balance sheets, Liabilities	0	0
Net amount of derivative assets after application of master netting agreements and cash collateral	112	12
Net amount of derivative liabilities after application of master netting agreements and cash collateral	19	23
Estimated fair value of derivative assets presented in the consolidated balance sheets	1,642	2,436
Estimated fair value of derivative liabilities presented in the consolidated balance sheets	835	1,016
Netting [Member] | Over-the-counter [Member]
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset	1,598	2,436
Derivative Liability, Fair Value, Gross Liability	821	982
Gross estimated fair value of derivative assets	(682)	(838)
Gross estimated fair value of derivative liabilities	(682)	(838)
Cash collateral on derivative assets	(356)	(897)
Cash collateral on derivative liabilities	0	0
Securities collateral on derivative assets	(456)	(689)
Securities collateral on derivative liabilities	(120)	(121)
Netting [Member] | Exchange-cleared [Member]
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset	39	0
Derivative Liability, Fair Value, Gross Liability	14	0
Gross estimated fair value of derivative assets	(13)	0
Gross estimated fair value of derivative liabilities	(13)	0
Cash collateral on derivative assets	(23)	0
Cash collateral on derivative liabilities	0	0
Securities collateral on derivative assets	0	0
Securities collateral on derivative liabilities	(1)	0
Netting [Member] | Exchange-traded [Member]
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset	5	0
Derivative Liability, Fair Value, Gross Liability	0	34
Gross estimated fair value of derivative assets	0	0
Gross estimated fair value of derivative liabilities	0	0
Cash collateral on derivative assets	0	0
Cash collateral on derivative liabilities	0	(34)
Securities collateral on derivative assets	0	0
Securities collateral on derivative liabilities	$ 0	$ 0