Pledged Assets (Offsetting Assets and Liabilities) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Offsetting Assets and Liabilities [Line Items]
Document Period End Date	Dec. 31, 2018
Collateral Received	$ (4,733)
Assets [Member]
Offsetting Assets and Liabilities [Line Items]
Gross Amounts of Recognized Assets	22,086	$ 11,147
Gross Amount Offset in the Consolidated Balance Sheets	0
Net Amounts of Assets Presented in the Consolidated Balance Sheets	22,086	11,147
Financial Instruments	(17,276)	(9,705)
Collateral Received		(1,361)
Net Amount	77	81
Assets [Member] | ERROR in label resolution.
Offsetting Assets and Liabilities [Line Items]
Gross Amounts of Recognized Assets	163	156
Net Amounts of Assets Presented in the Consolidated Balance Sheets	163	156
Financial Instruments	0	(1)
Collateral Received	(158)	(82)
Net Amount	5	73
Assets [Member] | TBA and Forward Settling Agency Securities [Member]
Offsetting Assets and Liabilities [Line Items]
Gross Amounts of Recognized Assets	110	30
Net Amounts of Assets Presented in the Consolidated Balance Sheets	110	30
Financial Instruments	(40)	(22)
Net Amount	70	8
Assets [Member] | Reverse Repurchase Agreements [Member]
Offsetting Assets and Liabilities [Line Items]
Net Amount	2
Liability [Member]
Offsetting Assets and Liabilities [Line Items]
Gross Amounts of Recognized Assets	75,757	50,324
Gross Amount Offset in the Consolidated Balance Sheets	0
Net Amounts of Assets Presented in the Consolidated Balance Sheets	75,757	50,324
Financial Instruments	(17,276)	(9,705)
Collateral Received	(58,481)	(40,619)
Net Amount	0
Liability [Member] | Repurchase Agreements [Member]
Offsetting Assets and Liabilities [Line Items]
Net Amount	0	0
Liability [Member] | ERROR in label resolution.
Offsetting Assets and Liabilities [Line Items]
Gross Amounts of Recognized Assets	0	1
Net Amounts of Assets Presented in the Consolidated Balance Sheets	0	1
Financial Instruments	0	(1)
Collateral Received	0	0
Net Amount	0	0
Liability [Member] | TBA and Forward Settling Agency Securities [Member]
Offsetting Assets and Liabilities [Line Items]
Gross Amounts of Recognized Assets	40	27
Net Amounts of Assets Presented in the Consolidated Balance Sheets	40	27
Financial Instruments	(40)	(22)
Collateral Received	0	(5)
Net Amount	0	0
Reverse Repurchase Agreements [Member] | Assets [Member]
Offsetting Assets and Liabilities [Line Items]
Gross Amounts of Recognized Assets	21,813	10,961
Gross Amount Offset in the Consolidated Balance Sheets	0
Net Amounts of Assets Presented in the Consolidated Balance Sheets	21,813	10,961
Financial Instruments	(17,236)	(9,682)
Collateral Received	(4,575)	(1,279)
Repurchase Agreements [Member] | Liability [Member]
Offsetting Assets and Liabilities [Line Items]
Gross Amounts of Recognized Assets	75,717	50,296
Gross Amount Offset in the Consolidated Balance Sheets	0
Net Amounts of Assets Presented in the Consolidated Balance Sheets	75,717	50,296
Financial Instruments	(17,236)	(9,682)
Collateral Received	$ (58,481)	$ (40,614)