Derivative and Other Hedging Instruments (Remaining Interest Rate Swap Term) (Details) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) month	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($) month
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	$ 249	$ (795)	$ (1,070)	$ (1,116)
Interest Rate Swaption [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(1)	(23)	(12)	(27)
Receiver Swaption [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net				4
Interest Rate Swap [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	153	(966)	(1,208)	(1,278)
Derivative Liability, Notional Amount	$ 34,150		$ 34,150		$ 40,525
Average Fixed Pay Rate	1.52%		1.52%		1.89%
Average Maturity (Years)			3 years 5 months 23 days		4 years 7 months 1 day
Less Than or Equal to One Year [Member] | Interest Rate Swaption [Member]
Cost			$ 44		$ 74
Fair Value	$ 6		$ 6		$ 17
Average Months to Expiration | month			5		4
Derivative Liability, Notional Amount	$ 700		$ 700		$ 2,150
Average Fixed Pay Rate	3.10%		3.10%		3.51%
Average Maturity (Years)			7 years 2 days		6 years 11 months 15 days
Less Than or Equal to One Year [Member] | Payer Swaption [Member]
Average Receive Rate (LIBOR)			3M		3M
Greater Than One Year and Less Than or Equal to Three Years [Member] | Interest Rate Swap [Member]
Derivative Liability, Notional Amount	$ 19,025		$ 19,025		$ 14,775
Average Fixed Pay Rate	1.05%		1.05%		1.06%
Average Maturity (Years)			1 year 4 months 24 days		1 year 7 months 2 days
Other Derivatives [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	$ 1		$ 15
Interest Only And Principal Only Strip [Member]
Unrealized Gain (Loss) on Securities	$ (6)	$ 10	$ 5	$ 14
Interest Rate Swaps Excluding Forward Starting [Member] | Interest Rate Swap [Member]
Average Fixed Pay Rate	1.40%		1.40%		1.75%