Investment Securities (Components Of Investment Securities) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Document Period End Date	Dec. 31, 2018
Agency securities transferred to consolidated VIEs	$ 436	$ 662
Debt Securities, Trading	60,692	23,427
Debt Securities, Trading and Available-for-sale	84,287	57,080
Amortized cost	$ 85,569	$ 57,466
Weighted average coupon	3.86%	3.71%
Weighted average yield	3.31%	2.89%
Future Prepayment Rate Assumption Of Investment Portfolio	8.00%	8.00%
Available-for-sale Securities [Member]
Schedule of Investments [Line Items]
Par value	$ 23,295	$ 32,460
Unamortized discount	(12)	(28)
Unamortized premium	1,255	1,566
Amortized cost	24,538	33,998
Gross unrealized gains	7	121
Gross unrealized losses	(950)	(466)
Total available-for-sale securities, at fair value	23,595	33,653
Trading Securities [Member]
Schedule of Investments [Line Items]
Unamortized discount	(103)	(35)
Unamortized premium	1,736	1,159
Debt Securities, Trading, Unrealized Gain	303	71
Debt Securities, Trading, Unrealized Loss	642	112
Trading Securities, Cost	61,031	23,468
Trading Securities Par	59,398	22,344
Fannie Mae [Member]
Schedule of Investments [Line Items]
Debt Securities, Trading	40,267	14,191
Debt Securities, Trading and Available-for-sale	$ 58,078	$ 39,258
Weighted average coupon	3.82%	3.67%
Weighted average yield	3.28%	2.84%
Fannie Mae [Member] \| Available-for-sale Securities [Member]
Schedule of Investments [Line Items]
Par value	$ 17,591	$ 24,200
Unamortized discount	(10)	(25)
Unamortized premium	912	1,119
Amortized cost	18,493	25,294
Gross unrealized gains	4	98
Gross unrealized losses	(686)	(325)
Total available-for-sale securities, at fair value	17,811	25,067
Fannie Mae [Member] \| Trading Securities [Member]
Schedule of Investments [Line Items]
Unamortized discount	(78)	(34)
Unamortized premium	1,055	711
Debt Securities, Trading, Unrealized Gain	223	26
Debt Securities, Trading, Unrealized Loss	386	70
Trading Securities, Cost	40,430	14,235
Trading Securities Par	39,453	13,558
Freddie Mac [Member]
Schedule of Investments [Line Items]
Debt Securities, Trading	18,871	8,331
Debt Securities, Trading and Available-for-sale	$ 24,623	$ 16,875
Weighted average coupon	3.87%	3.73%
Weighted average yield	3.28%	2.87%
Freddie Mac [Member] \| Available-for-sale Securities [Member]
Schedule of Investments [Line Items]
Par value	$ 5,673	$ 8,219
Unamortized discount	(2)	(3)
Unamortized premium	343	447
Amortized cost	6,014	8,663
Gross unrealized gains	2	22
Gross unrealized losses	(264)	(141)
Total available-for-sale securities, at fair value	5,752	8,544
Freddie Mac [Member] \| Trading Securities [Member]
Schedule of Investments [Line Items]
Unamortized discount	(9)	0
Unamortized premium	638	415
Debt Securities, Trading, Unrealized Gain	57	2
Debt Securities, Trading, Unrealized Loss	243	42
Trading Securities, Cost	19,057	8,371
Trading Securities Par	18,428	7,956
Ginnie Mae [Member]
Schedule of Investments [Line Items]
Debt Securities, Trading	0	0
Debt Securities, Trading and Available-for-sale	$ 26	$ 35
Weighted average coupon	3.37%	2.84%
Weighted average yield	2.04%	2.02%
Ginnie Mae [Member] \| Available-for-sale Securities [Member]
Schedule of Investments [Line Items]
Par value	$ 25	$ 34
Unamortized discount	0	0
Unamortized premium	0	0
Amortized cost	25	34
Gross unrealized gains	1	1
Gross unrealized losses	0	0
Total available-for-sale securities, at fair value	26	35
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	0	0
Trading Securities Par	0	0
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Investments [Line Items]
Debt Securities, Trading	260	0
Debt Securities, Trading and Available-for-sale	$ 266	$ 7
Weighted average coupon	3.83%	2.50%
Weighted average yield	4.22%	3.08%
Mortgage-backed Securities, Issued by Private Enterprises [Member] \| Available-for-sale Securities [Member]
Schedule of Investments [Line Items]
Par value	$ 6	$ 7
Unamortized discount	0	0
Unamortized premium	0	0
Amortized cost	6	7
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Total available-for-sale securities, at fair value	6	7
Mortgage-backed Securities, Issued by Private Enterprises [Member] \| Trading Securities [Member]
Schedule of Investments [Line Items]
Unamortized discount	(10)	0
Unamortized premium	0	0
Debt Securities, Trading, Unrealized Gain	2	0
Debt Securities, Trading, Unrealized Loss	0	0
Trading Securities, Cost	258	0
Trading Securities Par	268	0
Commercial Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Debt Securities, Trading	282	29
Debt Securities, Trading and Available-for-sale	$ 282	$ 29
Weighted average coupon	4.58%	6.55%
Weighted average yield	4.68%	7.30%
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	(6)	(1)
Unamortized premium	5	0
Debt Securities, Trading, Unrealized Gain	3	1
Debt Securities, Trading, Unrealized Loss	1	0
Trading Securities, Cost	280	28
Trading Securities Par	281	29
Credit Risk Transfer Securities [Member]
Schedule of Investments [Line Items]
Debt Securities, Trading	1,012	876
Debt Securities, Trading and Available-for-sale	$ 1,012	$ 876
Weighted average coupon	5.86%	5.26%
Weighted average yield	5.16%	5.19%
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	0	0
Unamortized premium	38	33
Debt Securities, Trading, Unrealized Gain	18	42
Debt Securities, Trading, Unrealized Loss	12	0
Trading Securities, Cost	1,006	834
Trading Securities Par	968	801
Credit Risk Transfer Securities [Member] \| Securities Remeasured at Fair Value [Member]
Schedule of Investments [Line Items]
Debt Securities, Trading and Available-for-sale	$ 1,012	$ 876

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