Derivative Instruments and Hedging Activities (Fair Value Amounts of Derivative Instruments Reported on our Condensed Consolidated Balance Sheet) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 39	$ 95
Derivative Liability, Fair Value, Gross Liability	41	95
Derivative, Notional Amount	23,762	34,166
Interest Receivable	550	476
Derivative assets in net asset position [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	38	87
Interest Receivable	0	7
Derivative assets in net liability positions [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		8
Interest Receivable	0	1
Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0	20
Derivative Liability, Fair Value, Gross Liability	1	21
Derivative, Notional Amount	7,051	4,902
Not Designated as Hedging Instrument [Member] | Other Trading [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	39	75
Derivative Liability, Fair Value, Gross Liability	40	74
Derivative, Notional Amount	16,711	29,264
Interest Rate Swap [Member] | Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0	19
Derivative Liability, Fair Value, Gross Liability	0	21
Derivative, Notional Amount	6,915	4,731
Foreign Exchange Forward [Member] | Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0	1
Derivative Liability, Fair Value, Gross Liability	1	0
Derivative, Notional Amount	136	171
Interest Rate Contract [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	39	73
Derivative Liability, Fair Value, Gross Liability	39	73
Derivative, Notional Amount	16,587	29,172
Interest Rate Contract [Member] | Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Liability	0	0
Derivative, Notional Amount	0	137
Interest Rate Contract [Member] | Not Designated as Hedging Instrument [Member] | Futures and Forwards [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Liability	0	0
Derivative, Notional Amount	23	0
Interest Rate Contract [Member] | Not Designated as Hedging Instrument [Member] | Written Options [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	1	0
Derivative Liability, Fair Value, Gross Liability	39	73
Derivative, Notional Amount	8,327	14,518
Interest Rate Contract [Member] | Not Designated as Hedging Instrument [Member] | Purchased Options [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	38	73
Derivative Liability, Fair Value, Gross Liability	0	0
Derivative, Notional Amount	8,237	14,517
Foreign Exchange Contract [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0	1
Derivative Liability, Fair Value, Gross Liability	1	0
Derivative, Notional Amount	124	92
Foreign Exchange Contract [Member] | Not Designated as Hedging Instrument [Member] | Foreign Exchange Forward [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0	1
Derivative Liability, Fair Value, Gross Liability	1	0
Derivative, Notional Amount	124	92
Equity Contract [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0	1
Derivative Liability, Fair Value, Gross Liability	0	1
Derivative, Notional Amount	0	0
Equity Contract [Member] | Not Designated as Hedging Instrument [Member] | Written Options [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Liability	0	1
Derivative, Notional Amount	0	0
Equity Contract [Member] | Not Designated as Hedging Instrument [Member] | Purchased Options [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0	1
Derivative Liability, Fair Value, Gross Liability	0	0
Derivative, Notional Amount	$ 0	$ 0