Investment Securities (Components Of Investment Securities) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Document Period End Date	Dec. 31, 2017
Agency securities transferred to consolidated VIEs	$ 662	$ 818
Amortized cost	57,466	46,866
Total agency MBS, at fair value	56,168	46,211
Mortgage-backed Securities Available-for-sale, Fair Value Disclosure	$ 57,080	$ 46,499
Weighted average coupon	3.71%	3.61%
Weighted average yield	2.89%	2.77%
Future Prepayment Rate Assumption Of Investment Portfolio	8.00%	8.00%
Mortgage Backed Securities Amortized Cost	$ 57,466	$ 46,866
Available-for-sale Securities [Member]
Schedule of Investments [Line Items]
Par value	32,460	44,397
Unamortized discount	(28)	(46)
Unamortized premium	1,566	2,063
Amortized cost	33,998	46,414
Gross unrealized gains	121	225
Gross unrealized losses	(466)	(622)
Total available-for-sale securities, at fair value	33,653	46,017
Agency securities remeasured at fair value through earnings [Member]
Schedule of Investments [Line Items]
Securities Remeasured at Fair Value through earnings, Amortized Cost	23,468	452
Securities Remeasured at Fair Value Through Earnings, Gross Unrealized Gain	71	34
Securities Remeasured at Fair Value Through Earnings, Gross Unrealized Loss	(112)	(4)
Securities Remeasured at Fair Value Through Earnings, Debt Securities	23,427	482
Securities remeasured at fair value through earnings, Par Value	22,344	352
Securities Remeasured at Fair Value [Member]
Schedule of Investments [Line Items]
Unamortized discount	(35)	(36)
Unamortized premium	1,159	136
Fannie Mae [Member]
Schedule of Investments [Line Items]
Total agency MBS, at fair value	$ 39,258	$ 35,732
Weighted average coupon	3.67%	3.59%
Weighted average yield	2.84%	2.77%
Fannie Mae [Member] | Available-for-sale Securities [Member]
Schedule of Investments [Line Items]
Par value	$ 24,200	$ 34,244
Unamortized discount	(25)	(43)
Unamortized premium	1,119	1,518
Amortized cost	25,294	35,719
Gross unrealized gains	98	176
Gross unrealized losses	(325)	(442)
Total available-for-sale securities, at fair value	25,067	35,453
Fannie Mae [Member] | Agency securities remeasured at fair value through earnings [Member]
Schedule of Investments [Line Items]
Securities Remeasured at Fair Value through earnings, Amortized Cost	14,235	254
Securities Remeasured at Fair Value Through Earnings, Gross Unrealized Gain	26	28
Securities Remeasured at Fair Value Through Earnings, Gross Unrealized Loss	(70)	(3)
Securities Remeasured at Fair Value Through Earnings, Debt Securities	14,191	279
Securities remeasured at fair value through earnings, Par Value	13,558	171
Fannie Mae [Member] | Securities Remeasured at Fair Value [Member]
Schedule of Investments [Line Items]
Unamortized discount	(34)	(35)
Unamortized premium	711	118
Freddie Mac [Member]
Schedule of Investments [Line Items]
Total agency MBS, at fair value	$ 16,875	$ 10,434
Weighted average coupon	3.73%	3.67%
Weighted average yield	2.87%	2.72%
Freddie Mac [Member] | Available-for-sale Securities [Member]
Schedule of Investments [Line Items]
Par value	$ 8,219	$ 10,008
Unamortized discount	(3)	(3)
Unamortized premium	447	544
Amortized cost	8,663	10,549
Gross unrealized gains	22	48
Gross unrealized losses	(141)	(179)
Total available-for-sale securities, at fair value	8,544	10,418
Freddie Mac [Member] | Agency securities remeasured at fair value through earnings [Member]
Schedule of Investments [Line Items]
Securities Remeasured at Fair Value through earnings, Amortized Cost	8,371	14
Securities Remeasured at Fair Value Through Earnings, Gross Unrealized Gain	2	3
Securities Remeasured at Fair Value Through Earnings, Gross Unrealized Loss	(42)	(1)
Securities Remeasured at Fair Value Through Earnings, Debt Securities	8,331	16
Securities remeasured at fair value through earnings, Par Value	7,956	0
Freddie Mac [Member] | Securities Remeasured at Fair Value [Member]
Schedule of Investments [Line Items]
Unamortized discount	0	0
Unamortized premium	415	14
Ginnie Mae [Member]
Schedule of Investments [Line Items]
Total agency MBS, at fair value	$ 35	$ 45
Weighted average coupon	2.84%	2.75%
Weighted average yield	2.02%	2.00%
Ginnie Mae [Member] | Available-for-sale Securities [Member]
Schedule of Investments [Line Items]
Par value	$ 34	$ 44
Unamortized discount	0	0
Unamortized premium	0	0
Amortized cost	34	44
Gross unrealized gains	1	1
Gross unrealized losses	0	0
Total available-for-sale securities, at fair value	35	45
Ginnie Mae [Member] | Agency securities remeasured at fair value through earnings [Member]
Schedule of Investments [Line Items]
Securities Remeasured at Fair Value through earnings, Amortized Cost	0	0
Securities Remeasured at Fair Value Through Earnings, Gross Unrealized Gain	0	0
Securities Remeasured at Fair Value Through Earnings, Gross Unrealized Loss	0	0
Securities Remeasured at Fair Value Through Earnings, Debt Securities	0	0
Securities remeasured at fair value through earnings, Par Value	0	0
Ginnie Mae [Member] | Securities Remeasured at Fair Value [Member]
Schedule of Investments [Line Items]
Unamortized discount	0	0
Unamortized premium	0	0
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Investments [Line Items]
Amortized cost	7	102
Total Securities	$ 7	$ 101
Weighted average coupon	2.50%	3.42%
Weighted average yield	3.08%	3.27%
Mortgage-backed Securities, Issued by Private Enterprises [Member] | Available-for-sale Securities [Member]
Schedule of Investments [Line Items]
Par value	$ 7	$ 101
Unamortized discount	0	0
Unamortized premium	0	1
Amortized cost	7	102
Gross unrealized gains	0	0
Gross unrealized losses	0	(1)
Total available-for-sale securities, at fair value	7	101
Mortgage-backed Securities, Issued by Private Enterprises [Member] | Agency securities remeasured at fair value through earnings [Member]
Schedule of Investments [Line Items]
Securities Remeasured at Fair Value through earnings, Amortized Cost	0	0
Securities Remeasured at Fair Value Through Earnings, Gross Unrealized Gain	0	0
Securities Remeasured at Fair Value Through Earnings, Gross Unrealized Loss	0	0
Securities Remeasured at Fair Value Through Earnings, Debt Securities	0	0
Securities remeasured at fair value through earnings, Par Value	0	0
Mortgage-backed Securities, Issued by Private Enterprises [Member] | Securities Remeasured at Fair Value [Member]
Schedule of Investments [Line Items]
Unamortized discount	0	0
Unamortized premium	0	0
Commercial Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Amortized cost	28	23
Total Securities	$ 29	$ 23
Weighted average coupon	6.55%	6.55%
Weighted average yield	7.30%	7.54%
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] | Agency securities remeasured at fair value through earnings [Member]
Schedule of Investments [Line Items]
Securities Remeasured at Fair Value through earnings, Amortized Cost	28	23
Securities Remeasured at Fair Value Through Earnings, Gross Unrealized Gain	1	0
Securities Remeasured at Fair Value Through Earnings, Gross Unrealized Loss	0	0
Securities Remeasured at Fair Value Through Earnings, Debt Securities	29	23
Securities remeasured at fair value through earnings, Par Value	29	24
Commercial Mortgage Backed Securities [Member] | Securities Remeasured at Fair Value [Member]
Schedule of Investments [Line Items]
Unamortized discount	(1)	(1)
Unamortized premium	0	0
Credit Risk Transfer Securities [Member]
Schedule of Investments [Line Items]
Trading Securities, Cost	$ 834	$ 161
Weighted average coupon	5.26%	5.25%
Weighted average yield	5.19%	6.28%
Trading Securities, Other	$ 876	$ 164
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] | Securities Remeasured at Fair Value [Member]
Schedule of Investments [Line Items]
Unamortized discount	0	0
Unamortized premium	33	4
Securities Remeasured at Fair Value through earnings, Amortized Cost	834	161
Securities Remeasured at Fair Value Through Earnings, Gross Unrealized Gain	42	3
Securities Remeasured at Fair Value Through Earnings, Gross Unrealized Loss	0	0
Securities Remeasured at Fair Value Through Earnings, Debt Securities	876	164
Securities remeasured at fair value through earnings, Par Value	801	157
Total Securities	$ 876	$ 164