Derivatives (Estimated Fair Value of Derivatives Assets and Liabilities after Master Netting Agreements and Cash) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset Excluding Accruals	$ 2,820	$ 2,291
Derivative Liability, Fair Value, Gross Liability Excluding Accruals	2,104	3,120
Estimated fair value of derivative assets presented in the consolidated balance sheets	2,820	2,291
Estimated fair value of derivative liabilities presented in the consolidated balance sheets	2,104	3,120
Net amount of derivative assets after application of master netting agreements and cash collateral	13	43
Net amount of derivative liabilities after application of master netting agreements and cash collateral	0	0
Over the Counter
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset Excluding Accruals	2,800	2,222
Derivative Liability, Fair Value, Gross Liability Excluding Accruals	2,102	3,080
Gross estimated fair value of derivative assets	(1,669)	(1,942)
Gross estimated fair value of derivative liabilities	(1,669)	(1,942)
Cash collateral on derivative assets	(1,038)	(247)
Cash collateral on derivative liabilities	0	0
Securities collateral on derivative assets	(83)	(31)
Securities collateral on derivative liabilities	(433)	(1,138)
Exchange Cleared
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset Excluding Accruals	20	69
Derivative Liability, Fair Value, Gross Liability Excluding Accruals	2	40
Gross estimated fair value of derivative assets	(2)	(1)
Gross estimated fair value of derivative liabilities	(2)	(1)
Cash collateral on derivative assets	(15)	(27)
Cash collateral on derivative liabilities	$ 0	$ (39)