Derivatives and Hedging Activities (Offsetting of Derivative Assets and Derivative Liabilities) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Offsetting Assets and Liabilities [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 67	$ 39
Derivative Liability, Fair Value, Gross Liability	456	202
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	162	63
Derivative Asset, Not Subject to Master Netting Arrangement	4	0
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	(447)	(201)
Derivative Liability, Not Subject to Master Netting Arrangement	0	0
Derivative assets, net	233	102
Derivative liabilities, net	9	1
Over the Counter [Member]
Offsetting Assets and Liabilities [Line Items]
Derivative Asset, Fair Value, Gross Asset	49	34
Derivative Liability, Fair Value, Gross Liability	447	199
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	(43)	(28)
Derivative Asset, Not Subject to Master Netting Arrangement	4	0
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	(438)	(198)
Derivative Liability, Not Subject to Master Netting Arrangement	0	0
Derivative assets, net	10	6
Derivative liabilities, net	9	1
Exchange Cleared [Member]
Offsetting Assets and Liabilities [Line Items]
Derivative Asset, Fair Value, Gross Asset	18	5
Derivative Liability, Fair Value, Gross Liability	9	3
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	205	91
Derivative Asset, Not Subject to Master Netting Arrangement	0	0
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	(9)	(3)
Derivative Liability, Not Subject to Master Netting Arrangement	0	0
Derivative assets, net	223	96
Derivative liabilities, net	$ 0	$ 0