Derivatives (Estimated Fair Value of Derivative Assets and Liabilities after Master Netting Agreements and Cash Collateral) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset Excluding Accruals	$ 2,533	$ 3,707
Derivative Liability, Fair Value, Gross Liability Excluding Accruals	3,128	3,834
Amounts offset in the consoilidated balance sheets, Assets	0	0
Amounts offset in the consoilidated balance sheets, Liabilities	0	0
Estimated fair value of derivative assets presented in the consolidated balance sheets	2,533	3,707
Estimated fair value of derivative liabilities presented in the consolidated balance sheets	3,128	3,834
Net amount of derivative assets after application of master netting agreements and cash collateral	7	157
Net amount of derivative liabilities after application of master netting agreements and cash collateral	8	0
Over the Counter [Member]
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset Excluding Accruals	2,502	3,394
Derivative Liability, Fair Value, Gross Liability Excluding Accruals	3,095	2,929
Gross estimated fair value of derivative assets	(2,050)	(2,231)
Gross estimated fair value of derivative liabilities	(2,050)	(2,231)
Cash collateral on derivative assets	(336)	(634)
Cash collateral on derivative liabilities	0	0
Securities collateral on derivative assets	(114)	(429)
Securities collateral on derivative liabilities	(1,037)	(698)
OTC-cleared
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset Excluding Accruals	31	267
Derivative Liability, Fair Value, Gross Liability Excluding Accruals	22	905
Gross estimated fair value of derivative assets	(2)	(165)
Gross estimated fair value of derivative liabilities	(2)	(165)
Cash collateral on derivative assets	(24)	(91)
Cash collateral on derivative liabilities	(19)	(740)
Securities collateral on derivative assets	0	0
Securities collateral on derivative liabilities	(1)	0
Exchange-traded
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset Excluding Accruals	0	46
Derivative Liability, Fair Value, Gross Liability Excluding Accruals	11	0
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	0
Securities collateral on derivative assets	0	0
Securities collateral on derivative liabilities	$ (11)	$ 0