Offsetting Assets and Liabilities (Offsetting Assets and Liabilities) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Resale agreements
Gross Assets	$ 8,465	$ 9,547
Gross Amounts Offset in the Condensed Consolidated Balance Sheets	0	0
Net Amounts Presented in the Condensed Consolidated Balance Sheets	8,465	9,547
Counterparty Offsetting	0	0
Collateral	(8,465)	(9,547)
Resale agreements, Net Amount	0	0
Securities borrowed
Gross Assets	441	393
Gross Amounts Offset in the Condensed Consolidated Balance Sheets	0	0
Net Amounts Presented in the Condensed Consolidated Balance Sheets	441	393
Counterparty Offsetting	(304)	(200)
Collateral	(136)	(189)
Net Amount	1	4
Total Gross Assets	8,906	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	8,906	9,940
Total Assets, Gross Amounts Not Offset in the Consolidated Balance Sheet, Counterparty Offsetting	(304)	(200)
Total Assets, Gross Amounts Not Offset in the Consolidated Balance Sheet, Collateral	(8,601)	(9,736)
Total Assets, Net Amount	1	4
Securities loaned
Gross Liabilities	1,869	1,996
Net Amounts Presented in the Condensed Consolidated Balance Sheets	1,869	1,996
Counterparty Offsetting	(304)	(200)
Collateral	(1,443)	(1,660)
Net Amount	122	136
Total Gross Liabilities	1,869	1,996
Gross Amounts Offset in the Condensed Consolidated Balance Sheets	0	0
Total Liabilities, Net Amounts Presented in the Consolidated Balance Sheet	1,869	1,996
Total Liabilities, Gross Amounts Not Offset in the Consolidated Balance Sheet, Counterparty Offsetting	(304)	(200)
Total Liabilities, Gross Amounts Not Offset in the Consolidated Balance Sheet, Collateral	(1,443)	(1,660)
Total Liabilities, Net Amount	122	136
Offsetting Assets [Line Items]
Fair value of client securities available to be pledged	$ 21,441	$ 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	$ 8,600	$ 9,800