Investment Securities (Components Of Investment Securities) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Mar. 31, 2020	Jun. 30, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Debt Securities, Trading		$ 59,700	$ 80,289
Debt Securities, Trading and Available-for-sale		77,143	100,442
Amortized cost		$ 73,828	$ 98,670
Weighted average coupon		3.71%	3.68%
Weighted average yield		2.64%	3.07%
Future Prepayment Rate Assumption Of Investment Portfolio	10.80%	16.60%
Available-for-sale Securities [Member]
Schedule of Investments [Line Items]
Par value		$ 15,937	$ 19,081
Unamortized discount		(7)	(12)
Unamortized premium		749	987
Amortized cost		16,679	20,056
Gross unrealized gains	$ 172	765
Gross unrealized losses	(75)	(1)
Total available-for-sale securities, at fair value		17,443	20,153
Trading Securities [Member]
Schedule of Investments [Line Items]
Unamortized discount		(42)	(85)
Unamortized premium		2,250	2,219
Debt Securities, Trading, Unrealized Gain	1,729	2,608
Debt Securities, Trading, Unrealized Loss	(54)	(57)
Trading Securities, Cost		57,149	78,614
Trading Securities Par		54,941	76,480
Fannie Mae [Member]
Schedule of Investments [Line Items]
Debt Securities, Trading		34,852	47,215
Debt Securities, Trading and Available-for-sale		$ 47,738	$ 62,309
Weighted average coupon		3.65%	3.62%
Weighted average yield		2.60%	3.03%
Fannie Mae [Member] \| Available-for-sale Securities [Member]
Schedule of Investments [Line Items]
Par value		$ 11,774	$ 14,301
Unamortized discount		(6)	(10)
Unamortized premium		538	711
Amortized cost		12,306	15,002
Gross unrealized gains	142	581
Gross unrealized losses	(50)	(1)
Total available-for-sale securities, at fair value		12,886	15,094
Fannie Mae [Member] \| Trading Securities [Member]
Schedule of Investments [Line Items]
Unamortized discount		(22)	(68)
Unamortized premium		1,323	1,218
Debt Securities, Trading, Unrealized Gain	991	1,471
Debt Securities, Trading, Unrealized Loss	(32)	(4)
Trading Securities, Cost		33,385	46,256
Trading Securities Par		32,084	45,106
Freddie Mac [Member]
Schedule of Investments [Line Items]
Debt Securities, Trading		23,533	31,519
Debt Securities, Trading and Available-for-sale		$ 28,073	$ 36,559
Weighted average coupon		3.80%	3.75%
Weighted average yield		2.68%	3.09%
Freddie Mac [Member] \| Available-for-sale Securities [Member]
Schedule of Investments [Line Items]
Par value		$ 4,147	$ 4,762
Unamortized discount		(1)	(2)
Unamortized premium		211	276
Amortized cost		4,357	5,036
Gross unrealized gains	29	183
Gross unrealized losses	(25)	0
Total available-for-sale securities, at fair value		4,540	5,040
Freddie Mac [Member] \| Trading Securities [Member]
Schedule of Investments [Line Items]
Unamortized discount		(2)	(2)
Unamortized premium		898	967
Debt Securities, Trading, Unrealized Gain	691	1,103
Debt Securities, Trading, Unrealized Loss	(18)	(3)
Trading Securities, Cost		22,433	30,846
Trading Securities Par		21,537	29,881
Ginnie Mae [Member]
Schedule of Investments [Line Items]
Debt Securities, Trading		4	0
Debt Securities, Trading and Available-for-sale		$ 21	$ 19
Weighted average coupon		3.80%	3.77%
Weighted average yield		2.23%	2.08%
Ginnie Mae [Member] \| Available-for-sale Securities [Member]
Schedule of Investments [Line Items]
Par value		$ 16	$ 18
Unamortized discount		0	0
Unamortized premium		0	0
Amortized cost		16	18
Gross unrealized gains	1	1
Gross unrealized losses	0	0
Total available-for-sale securities, at fair value		17	19
Ginnie Mae [Member] \| Trading Securities [Member]
Schedule of Investments [Line Items]
Unamortized discount		0	0
Unamortized premium		0	0
Debt Securities, Trading, Unrealized Gain	0	0
Debt Securities, Trading, Unrealized Loss	0	0
Trading Securities, Cost		4	0
Trading Securities Par		4	0
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Investments [Line Items]
Debt Securities, Trading		222	209
Debt Securities, Trading and Available-for-sale		$ 222	$ 209
Weighted average coupon		4.15%	4.05%
Weighted average yield		4.01%	4.39%
Mortgage-backed Securities, Issued by Private Enterprises [Member] \| Available-for-sale Securities [Member]
Schedule of Investments [Line Items]
Par value		$ 0	$ 0
Unamortized discount		0	0
Unamortized premium		0	0
Amortized cost		0	0
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Total available-for-sale securities, at fair value		0	0
Mortgage-backed Securities, Issued by Private Enterprises [Member] \| Trading Securities [Member]
Schedule of Investments [Line Items]
Unamortized discount		(10)	(10)
Unamortized premium		4	1
Debt Securities, Trading, Unrealized Gain	10	8
Debt Securities, Trading, Unrealized Loss	0	(9)
Trading Securities, Cost		223	199
Trading Securities Par		229	208
Commercial Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Debt Securities, Trading		377	370
Debt Securities, Trading and Available-for-sale		$ 377	$ 370
Weighted average coupon		4.20%	4.49%
Weighted average yield		4.11%	4.38%
Commercial Mortgage Backed Securities [Member] \| Available-for-sale Securities [Member]
Schedule of Investments [Line Items]
Par value		$ 0	$ 0
Unamortized discount		0	0
Unamortized premium		0	0
Amortized cost		0	0
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Total available-for-sale securities, at fair value		0	0
Commercial Mortgage Backed Securities [Member] \| Trading Securities [Member]
Schedule of Investments [Line Items]
Unamortized discount		(2)	(3)
Unamortized premium		7	7
Debt Securities, Trading, Unrealized Gain	19	23
Debt Securities, Trading, Unrealized Loss	(1)	(8)
Trading Securities, Cost		362	352
Trading Securities Par		357	348
Credit Risk Transfer Securities [Member]
Schedule of Investments [Line Items]
Debt Securities, Trading		712	976
Debt Securities, Trading and Available-for-sale		$ 712	$ 976
Weighted average coupon		3.46%	5.07%
Weighted average yield		2.94%	4.05%
Credit Risk Transfer Securities [Member] \| Available-for-sale Securities [Member]
Schedule of Investments [Line Items]
Par value		$ 0	$ 0
Unamortized discount		0	0
Unamortized premium		0	0
Amortized cost		0	0
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Total available-for-sale securities, at fair value		0	0
Credit Risk Transfer Securities [Member] \| Trading Securities [Member]
Schedule of Investments [Line Items]
Unamortized discount		(6)	(2)
Unamortized premium		18	26
Debt Securities, Trading, Unrealized Gain	18	3
Debt Securities, Trading, Unrealized Loss	$ (3)	(33)
Trading Securities, Cost		742	961
Trading Securities Par		730	937
Credit Risk Transfer Securities [Member] \| Securities Remeasured at Fair Value [Member]
Schedule of Investments [Line Items]
Debt Securities, Trading and Available-for-sale		$ 712	$ 976

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