Financial Instruments Subject to Off-Balance Sheet Credit Risk (Offsetting Assets and Liabilities) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Resale agreements
Gross Assets	$ 6,596	$ 9,547
Gross Amounts Offset in the Consolidated Balance Sheets	0	0
Net Amounts Presented in the Consolidated Balance Sheets	6,596	9,547
Counterparty Offsetting	0	0
Collateral	(6,596)	(9,547)
Net Amount	0	0
Securities borrowed
Gross Assets	222	393
Gross Amounts Offset in the Consolidated Balance Sheets	0	0
Net Amounts Presented in the Consolidated Balance Sheets	222	393
Counterparty Offsetting	(199)	(200)
Collateral	(22)	(189)
Net Amount	1	4
Total Gross Assets	6,818	9,940
Total Assets, Gross Amounts Offset in the Condensed Consolidated Balance Sheets	0	0
Total Assets, Net Amounts Presented in the Condensed Consolidated Balance Sheets	6,818	9,940
Total Assets, Gross Amounts Not Offset in the Consolidated Balance Sheet, Counterparty Offsetting	(199)	(200)
Total Assets, Gross Amounts Not Offset in the Consolidated Balance Sheet, Collateral	(6,618)	(9,736)
Total Assets, Net Amount	1	4
Securities loaned
Gross Liabilities	966	1,996
Gross Amounts Offset in the Consolidated Balance Sheets	0	0
Net Amounts Presented in the Consolidated Balance Sheets	966	1,996
Counterparty Offsetting	(199)	(200)
Collateral	(670)	(1,660)
Net Amount	97	136
Total Gross Liabilities	966	1,996
Total Liabilities, Net Amounts Presented in the Consolidated Balance Sheet	966	1,996
Total Liabilities, Gross Amounts Not Offset in the Consolidated Balance Sheet, Counterparty Offsetting	(199)	(200)
Total Liabilities, Gross Amounts Not Offset in the Consolidated Balance Sheet, Collateral	(670)	(1,660)
Total Liabilities, Net Amount	97	136
Offsetting Assets [Line Items]
Fair value of client securities available to be pledged	$ 25,905	$ 21,516
Resale And Repurchase Agreements [Member]
Offsetting Assets [Line Items]
Percentage of collateral to related assets	102.00%	102.00%
Fair value of client securities available to be pledged	$ 6,700	$ 9,800