Financial Instruments Subject to Off-Balance Sheet Credit Risk (Offsetting Assets and Liabilities) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Resale agreements
Gross Assets	$ 4,434	$ 6,596
Gross Amounts Offset in the Condensed Consolidated Balance Sheets	0	0
Net Amounts Presented in the Condensed Consolidated Balance Sheets	4,434	6,596
Counterparty Offsetting	0	0
Collateral	(4,434)	(6,596)
Net Amount	0	0
Securities borrowed
Gross Assets	241	222
Gross Amounts Offset in the Condensed Consolidated Balance Sheets	0	0
Net Amounts Presented in the Condensed Consolidated Balance Sheets	241	222
Counterparty Offsetting	(172)	(199)
Collateral	(68)	(22)
Net Amount	1	1
Total Gross Assets	4,675	6,818
Total Assets, Gross Amounts Offset in the Condensed Consolidated Balance Sheets	0	0
Total Assets, Net Amounts Presented in the Condensed Consolidated Balance Sheets	4,675	6,818
Total Assets, Gross Amounts Not Offset in the Consolidated Balance Sheet, Counterparty Offsetting	(172)	(199)
Total Assets, Gross Amounts Not Offset in the Consolidated Balance Sheet, Collateral	(4,502)	(6,618)
Total Assets, Net Amount	1	1
Securities loaned
Gross Liabilities	800	966
Gross Amounts Offset in the Condensed Consolidated Balance Sheets	0	0
Net Amounts Presented in the Condensed Consolidated Balance Sheets	800	966
Counterparty Offsetting	(172)	(199)
Collateral	(558)	(670)
Net Amount	70	97
Total Gross Liabilities	800	966
Total Liabilities, Net Amounts Presented in the Consolidated Balance Sheet	800	966
Total Liabilities, Gross Amounts Not Offset in the Consolidated Balance Sheet, Counterparty Offsetting	(172)	(199)
Total Liabilities, Gross Amounts Not Offset in the Consolidated Balance Sheet, Collateral	(558)	(670)
Total Liabilities, Net Amount	70	97
Offsetting Assets [Line Items]
Fair value of client securities available to be pledged	$ 27,296	$ 25,905
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	$ 4,500	$ 6,700