Derivative and Other Hedging Instruments (Summary Of Outstanding Interest Rate Swaps) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Gain Loss on Other Debt	$ (2)	$ 7	$ (8)	$ 16
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	249	(795)	$ (1,070)	(1,116)
Forward Contracts [Member] | Interest Rate Swap [Member]
Average Maturity (Years)			6 years 9 months 9 days		5 years 6 months 7 days
Derivative, Notional Amount	2,200		$ 2,200		$ 4,500
Weighted Average Forward Start Date			6 months 6 days		8 months 10 days
Interest Rate Swap [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	153	(966)	$ (1,208)	(1,278)
Notional Amount	$ 34,150		$ 34,150		$ 40,525
Average Fixed Pay Rate	1.52%		1.52%		1.89%
Average Receive Rate	0.78%		0.78%		0.40%
Net Estimated Fair Value	$ (937)		$ (937)		$ (889)
Average Maturity (Years)			3 years 5 months 23 days		4 years 7 months 1 day
Greater Than One Year and Less Than or Equal to Three Years [Member] | Interest Rate Swap [Member]
Notional Amount	$ 19,025		$ 19,025		$ 14,775
Average Fixed Pay Rate	1.05%		1.05%		1.06%
Average Receive Rate	0.79%		0.79%		0.40%
Net Estimated Fair Value	$ (64)		$ (64)		$ (23)
Average Maturity (Years)			1 year 4 months 24 days		1 year 7 months 2 days
Greater Than Three Years and Less Than or Equal to Five Years [Member] | Interest Rate Swap [Member]
Notional Amount	$ 5,050		$ 5,050		$ 9,950
Average Fixed Pay Rate	1.55%		1.55%		2.03%
Average Receive Rate	0.76%		0.76%		0.40%
Net Estimated Fair Value	$ (89)		$ (89)		$ (203)
Average Maturity (Years)			3 years 6 months 14 days		3 years 11 months 22 days
Greater Than Five Years and Less than or Equal to Seven Years [Member] | Interest Rate Swap [Member]
Notional Amount	$ 7,075		$ 7,075		$ 7,175
Average Fixed Pay Rate	2.23%		2.23%		2.47%
Average Receive Rate	0.76%		0.76%		0.44%
Net Estimated Fair Value	$ (405)		$ (405)		$ (230)
Average Maturity (Years)			5 years 9 months 27 days		6 years 1 month 16 days
Greater Than Seven Years and Less than or Equal to Ten Years [Member] | Interest Rate Swap [Member]
Notional Amount	$ 1,825		$ 1,825		$ 7,450
Average Fixed Pay Rate	2.46%		2.46%		2.57%
Average Receive Rate	0.83%		0.83%		0.39%
Net Estimated Fair Value	$ (158)		$ (158)		$ (342)
Average Maturity (Years)			8 years 7 months 20 days		8 years 3 months 4 days
Greater Than Ten Years [Member] | Interest Rate Swap [Member]
Notional Amount	$ 1,175		$ 1,175		$ 1,175
Average Fixed Pay Rate	3.20%		3.20%		3.20%
Average Receive Rate	0.80%		0.80%		0.39%
Net Estimated Fair Value	$ (221)		$ (221)		$ (91)
Average Maturity (Years)			13 years 11 months 22 days		14 years 8 months 22 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