Financial Instruments Subject to Off-Balance Sheet Credit Risk (Offsetting Assets and Liabilities) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Resale agreements
Gross Assets	$ 17,044	$ 9,028
Gross Amounts Offset in the Condensed Consolidated Balance Sheets	0	0
Net Amounts Presented in the Condensed Consolidated Balance Sheets	17,044	9,028
Counterparty Offsetting	0	0
Collateral	(17,044)	(9,028)
Net Amount	0	0
Securities borrowed
Gross Assets	78	735
Gross Amounts Offset in the Condensed Consolidated Balance Sheets	0	0
Net Amounts Presented in the Condensed Consolidated Balance Sheets	78	735
Counterparty Offsetting	(71)	(730)
Collateral	(6)	(5)
Net Amount	1	0
Total Gross Assets	17,122	9,763
Total Assets, Gross Amounts Offset in the Condensed Consolidated Balance Sheets	0	0
Total Assets, Net Amounts Presented in the Condensed Consolidated Balance Sheets	17,122	9,763
Total Assets, Gross Amounts Not Offset in the Consolidated Balance Sheet, Counterparty Offsetting	(71)	(730)
Total Assets, Gross Amounts Not Offset in the Consolidated Balance Sheet, Collateral	(17,050)	(9,033)
Total Assets, Net Amount	1	0
Securities loaned
Gross Liabilities	1,335	1,251
Gross Amounts Offset in the Condensed Consolidated Balance Sheets	0	0
Net Amounts Presented in the Condensed Consolidated Balance Sheets	1,335	1,251
Counterparty Offsetting	(71)	(730)
Collateral	(1,106)	(445)
Net Amount	158	76
Total Gross Liabilities	1,335	1,251
Total Liabilities, Net Amounts Presented in the Consolidated Balance Sheet	1,335	1,251
Total Liabilities, Gross Amounts Not Offset in the Consolidated Balance Sheet, Counterparty Offsetting	(71)	(730)
Total Liabilities, Gross Amounts Not Offset in the Consolidated Balance Sheet, Collateral	(1,106)	(445)
Total Liabilities, Net Amount	158	76
Offsetting Assets [Line Items]
Fair value of client securities available to be pledged	22,896	26,685
Resale And Repurchase Agreements [Member]
Offsetting Assets [Line Items]
Fair value of client securities available to be pledged	$ 17,200	$ 9,200