Investment Securities (Components Of Investment Securities) (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Agency securities transferred to consolidated VIEs	$ 737	$ 818
Amortized cost	46,744	46,866
Total agency MBS, at fair value	45,963	46,211
Mortgage-backed Securities Available-for-sale, Fair Value Disclosure	$ 46,566	$ 46,499
Weighted average coupon	3.70%	3.61%
Weighted average yield	2.84%	2.77%
Future Prepayment Rate Assumption Of Investment Portfolio	9.00%	8.00%
Mortgage Backed Securities Amortized Cost	$ 46,744	$ 46,866
Available-for-sale Securities [Member]
Schedule of Investments [Line Items]
Par value	35,581	44,397
Unamortized discount	(30)	(46)
Unamortized premium	1,709	2,063
Amortized cost	37,260	46,414
Gross unrealized gains	210	225
Gross unrealized losses	(440)	(622)
Total available-for-sale securities, at fair value	37,030	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	9,484	452
Securities Remeasured at Fair Value Through Earnings, Gross Unrealized Gain	68	34
Securities Remeasured at Fair Value Through Earnings, Gross Unrealized Loss	(16)	(4)
Securities Remeasured at Fair Value Through Earnings, Debt Securities	9,536	482
Securities remeasured at fair value through earnings, Par Value	9,012	352
Securities Remeasured at Fair Value [Member]
Schedule of Investments [Line Items]
Unamortized discount	(39)	(36)
Unamortized premium	511	136
Fannie Mae [Member]
Schedule of Investments [Line Items]
Total agency MBS, at fair value	$ 36,069	$ 35,732
Weighted average coupon	3.68%	3.59%
Weighted average yield	2.83%	2.77%
Fannie Mae [Member] | Available-for-sale Securities [Member]
Schedule of Investments [Line Items]
Par value	$ 26,924	$ 34,244
Unamortized discount	(27)	(43)
Unamortized premium	1,242	1,518
Amortized cost	28,139	35,719
Gross unrealized gains	164	176
Gross unrealized losses	(305)	(442)
Total available-for-sale securities, at fair value	27,998	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	8,042	254
Securities Remeasured at Fair Value Through Earnings, Gross Unrealized Gain	42	28
Securities Remeasured at Fair Value Through Earnings, Gross Unrealized Loss	(13)	(3)
Securities Remeasured at Fair Value Through Earnings, Debt Securities	8,071	279
Securities remeasured at fair value through earnings, Par Value	7,648	171
Fannie Mae [Member] | Securities Remeasured at Fair Value [Member]
Schedule of Investments [Line Items]
Unamortized discount	(38)	(35)
Unamortized premium	432	118
Freddie Mac [Member]
Schedule of Investments [Line Items]
Total agency MBS, at fair value	$ 9,854	$ 10,434
Weighted average coupon	3.70%	3.67%
Weighted average yield	2.74%	2.72%
Freddie Mac [Member] | Available-for-sale Securities [Member]
Schedule of Investments [Line Items]
Par value	$ 8,610	$ 10,008
Unamortized discount	(3)	(3)
Unamortized premium	467	544
Amortized cost	9,074	10,549
Gross unrealized gains	45	48
Gross unrealized losses	(135)	(179)
Total available-for-sale securities, at fair value	8,984	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	870	14
Securities Remeasured at Fair Value Through Earnings, Gross Unrealized Gain	3	3
Securities Remeasured at Fair Value Through Earnings, Gross Unrealized Loss	(3)	(1)
Securities Remeasured at Fair Value Through Earnings, Debt Securities	870	16
Securities remeasured at fair value through earnings, Par Value	811	0
Freddie Mac [Member] | Securities Remeasured at Fair Value [Member]
Schedule of Investments [Line Items]
Unamortized discount	0	0
Unamortized premium	59	14
Ginnie Mae [Member]
Schedule of Investments [Line Items]
Total agency MBS, at fair value	$ 40	$ 45
Weighted average coupon	2.76%	2.75%
Weighted average yield	2.00%	2.00%
Ginnie Mae [Member] | Available-for-sale Securities [Member]
Schedule of Investments [Line Items]
Par value	$ 39	$ 44
Unamortized discount	0	0
Unamortized premium	0	0
Amortized cost	39	44
Gross unrealized gains	1	1
Gross unrealized losses	0	0
Total available-for-sale securities, at fair value	40	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	8	102
Total Securities	$ 8	$ 101
Weighted average coupon	2.50%	3.42%
Weighted average yield	3.00%	3.27%
Mortgage-backed Securities, Issued by Private Enterprises [Member] | Available-for-sale Securities [Member]
Schedule of Investments [Line Items]
Par value	$ 8	$ 101
Unamortized discount	0	0
Unamortized premium	0	1
Amortized cost	8	102
Gross unrealized gains	0	0
Gross unrealized losses	0	(1)
Total available-for-sale securities, at fair value	8	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	23	23
Total Securities	$ 23	$ 23
Weighted average coupon	6.55%	6.55%
Weighted average yield	7.53%	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	23	23
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	23	23
Securities remeasured at fair value through earnings, Par Value	24	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	$ 549	$ 161
Weighted average coupon	5.14%	5.25%
Weighted average yield	5.31%	6.28%
Trading Securities, Other	$ 572	$ 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	20	4
Securities Remeasured at Fair Value through earnings, Amortized Cost	549	161
Securities Remeasured at Fair Value Through Earnings, Gross Unrealized Gain	23	3
Securities Remeasured at Fair Value Through Earnings, Gross Unrealized Loss	0	0
Securities Remeasured at Fair Value Through Earnings, Debt Securities	572	164
Securities remeasured at fair value through earnings, Par Value	529	157
Total Securities	$ 572	$ 164