Derivative and Other Hedging Instruments (Summary Of Outstanding Interest Rate Swaps) (Details) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2020 USD ($) month	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($) month
Interest Rate Swaps Linked to Overnight Index Swap Rate	69.00%		86.00%
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	$ (3,180)	$ (996)
Interest Rate Swaps Linked to 3-Month LIBOR	5.00%		11.00%
Interest Rate Swaps Linked to Secured Overnight Financing Rate	26.00%		3.00%
Payer Swaption [Member]
Interest Rate Derivative Not Designated As Hedging Instruments Receive Rate	3M		3M
Interest Rate Swaption [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	$ (134)	(27)
Notional Amount	$ 9,550		$ 8,850
Average Fixed Pay Rate	2.23%		2.34%
Options At Cost	$ 182		$ 176
Interest Rate Derivatives, at Fair Value, Net	$ 21		$ 126
Cash Flow Hedges Derivative Instruments Not Designated As Hedging Instruments Average Months To Expiration | month	10		11
Average Maturity (Years)	9 years 6 months		9 years 6 months
Interest Rate Swap [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	$ (2,795)	$ (596)
Notional Amount	$ 46,475		$ 79,075
Average Fixed Pay Rate	0.94%		1.29%
Average Receive Rate	0.15%		1.59%
Average Maturity (Years)	4 years 6 months		2 years 8 months 12 days
Less Than or Equal to One Year [Member] | Payer Swaption [Member]
Interest Rate Derivative Not Designated As Hedging Instruments Receive Rate	3M		3M
Less Than or Equal to One Year [Member] | Interest Rate Swaption [Member]
Notional Amount	$ 6,350		$ 5,650
Average Fixed Pay Rate	2.14%		2.26%
Options At Cost	$ 132		$ 123
Interest Rate Derivatives, at Fair Value, Net	$ 11		$ 80
Cash Flow Hedges Derivative Instruments Not Designated As Hedging Instruments Average Months To Expiration | month	7		8
Average Maturity (Years)	9 years 3 months 18 days		9 years 3 months 18 days
Greater Than One Year and Less Than or Equal to Two Years [Member] | Payer Swaption [Member]
Interest Rate Derivative Not Designated As Hedging Instruments Receive Rate	3M		3M
Greater Than One Year and Less Than or Equal to Two Years [Member] | Interest Rate Swaption [Member]
Notional Amount	$ 3,200		$ 3,200
Average Fixed Pay Rate	2.41%		2.50%
Options At Cost	$ 50		$ 53
Interest Rate Derivatives, at Fair Value, Net	$ 10		$ 46
Cash Flow Hedges Derivative Instruments Not Designated As Hedging Instruments Average Months To Expiration | month	16		16
Average Maturity (Years)	10 years		10 years
Greater Than One Year and Less Than or Equal to Three Years [Member] | Interest Rate Swap [Member]
Notional Amount	$ 16,650		$ 59,700
Average Fixed Pay Rate	0.86%		1.30%
Average Receive Rate	0.21%		1.58%
Average Maturity (Years)	2 years 6 months		1 year 7 months 6 days
Greater Than Three Years and Less Than or Equal to Five Years [Member] | Interest Rate Swap [Member]
Notional Amount	$ 17,350		$ 9,850
Average Fixed Pay Rate	0.91%		1.17%
Average Receive Rate	0.06%		1.55%
Average Maturity (Years)	4 years 1 month 6 days		3 years 9 months 18 days
Greater Than Five Years and Less than or Equal to Seven Years [Member] | Interest Rate Swap [Member]
Notional Amount	$ 7,600		$ 5,650
Average Fixed Pay Rate	1.08%		1.34%
Average Receive Rate	0.21%		1.70%
Average Maturity (Years)	6 years 1 month 6 days		6 years 4 months 24 days
Greater Than Seven Years and Less than or Equal to Ten Years [Member] | Interest Rate Swap [Member]
Notional Amount	$ 3,900		$ 2,850
Average Fixed Pay Rate	1.05%		1.36%
Average Receive Rate	0.08%		1.58%
Average Maturity (Years)	9 years 1 month 6 days		8 years 10 months 24 days
Greater Than Ten Years [Member] | Interest Rate Swap [Member]
Notional Amount	$ 975		$ 1,025
Average Fixed Pay Rate	1.30%		1.64%
Average Receive Rate	0.45%		1.78%
Average Maturity (Years)	16 years		15 years 4 months 24 days