Derivatives (Estimated Fair Value of Derivative Assets and Liabilities after Master Netting Agreements and Cash Collateral) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset Excluding Accruals	$ 8,626	$ 12,284
Derivative Liability, Fair Value, Gross Liability Excluding Accruals	4,290	6,878
Amounts offset in the consolidated balance sheet, Assets	0	0
Amounts offset in the consolidated balance sheet, Liabilities	0	0
Estimated fair value of derivative assets presented in the consolidated balance sheets	8,626	12,284
Estimated fair value of derivative liabilities presented in the consolidated balance sheets	4,290	6,878
Net amount of derivative assets after application of master netting agreements and cash collateral	305	370
Net amount of derivative liabilities after application of master netting agreements and cash collateral	57	38
Over the Counter [Member]
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset Excluding Accruals	7,955	9,976
Derivative Liability, Fair Value, Gross Liability Excluding Accruals	4,059	5,721
Gross estimated fair value of derivative assets	(2,528)	(3,787)
Gross estimated fair value of derivative liabilities	(2,528)	(3,787)
Cash collateral on derivative assets	(4,169)	(4,244)
Cash collateral on derivative liabilities	0	(84)
Securities collateral on derivative assets	(1,004)	(1,640)
Securities collateral on derivative liabilities	(1,474)	(1,818)
Exchange Traded [Member]
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset Excluding Accruals	22	33
Derivative Liability, Fair Value, Gross Liability Excluding Accruals	8	15
Gross estimated fair value of derivative assets	(1)	(5)
Gross estimated fair value of derivative liabilities	(1)	(5)
Cash collateral on derivative assets	0	0
Cash collateral on derivative liabilities	(5)	(9)
Securities collateral on derivative assets	0	0
Securities collateral on derivative liabilities	(2)	0
Cleared [Member]
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset Excluding Accruals	649	2,275
Derivative Liability, Fair Value, Gross Liability Excluding Accruals	223	1,142
Gross estimated fair value of derivative assets	(35)	(903)
Gross estimated fair value of derivative liabilities	(35)	(903)
Cash collateral on derivative assets	(584)	(1,335)
Cash collateral on derivative liabilities	(179)	(234)
Securities collateral on derivative assets	0	0
Securities collateral on derivative liabilities	$ (9)	$ 0