Pledged Assets (Assets Pledged as Collateral) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Offsetting Assets and Liabilities [Line Items]
Pledged Financial Instruments, Not Separately Reported, Mortgage-Related Securities Available-for-sale or Held-for-investment	$ 68,370	$ 92,547
Retained Interests in Consolidated VIE's Pledged as Collateral Under Repurchase Agreements	113	144
Accrued interest on pledged securities	197	270
Restricted cash	1,978	451
Total Fair Value of Securities Pledged and Accrued Interest	71,319
Total Fair Value Of Agency Securities Pledged And Accrued Interest		94,634
Repurchase Agreements [Member]
Offsetting Assets and Liabilities [Line Items]
Pledged Financial Instruments, Not Separately Reported, Mortgage-Related Securities Available-for-sale or Held-for-investment	64,059	92,142
Derivative [Member]
Offsetting Assets and Liabilities [Line Items]
Pledged Financial Instruments, Not Separately Reported, Mortgage-Related Securities Available-for-sale or Held-for-investment	157	404
Accrued interest on pledged securities		1
Restricted cash	938	340
Total Fair Value of Securities Pledged and Accrued Interest	1,303
Total Fair Value Of Agency Securities Pledged And Accrued Interest		745
Repurchase Agreements [Member]
Offsetting Assets and Liabilities [Line Items]
Accrued interest on pledged securities	195	267
Total Fair Value of Securities Pledged and Accrued Interest	69,605
Total Fair Value Of Agency Securities Pledged And Accrued Interest		93,282
Variable Interest Entity, Primary Beneficiary [Member]
Offsetting Assets and Liabilities [Line Items]
Pledged Financial Instruments, Not Separately Reported, Mortgage-Related Securities Available-for-sale or Held-for-investment	358	371
Accrued interest on pledged securities	1	1
Total Fair Value of Securities Pledged and Accrued Interest	359
Total Fair Value Of Agency Securities Pledged And Accrued Interest		372
Under Prime Broker Agreements [Member]
Offsetting Assets and Liabilities [Line Items]
Pledged Financial Instruments, Not Separately Reported, Mortgage-Related Securities Available-for-sale or Held-for-investment	52	206
Accrued interest on pledged securities	0	1
Total Fair Value of Securities Pledged and Accrued Interest	52
Total Fair Value Of Agency Securities Pledged And Accrued Interest		235
Includes Sold But Not Yet Settled Securities [Member]
Offsetting Assets and Liabilities [Line Items]
Pledged Financial Instruments, Not Separately Reported, Mortgage-Related Securities Available-for-sale or Held-for-investment	64,626	93,123
Credit Risk Transfer Securities [Member]
Offsetting Assets and Liabilities [Line Items]
Pledged Financial Instruments, Not Separately Reported, Mortgage-Related Securities Available-for-sale or Held-for-investment	360	309
Credit Risk Transfer Securities [Member] \| Repurchase Agreements [Member]
Offsetting Assets and Liabilities [Line Items]
Pledged Financial Instruments, Not Separately Reported, Mortgage-Related Securities Available-for-sale or Held-for-investment	360	309
Credit Risk Transfer Securities [Member] \| Derivative [Member]
Offsetting Assets and Liabilities [Line Items]
Pledged Financial Instruments, Not Separately Reported, Mortgage-Related Securities Available-for-sale or Held-for-investment	0	0
Non-Agency [Member]
Offsetting Assets and Liabilities [Line Items]
Pledged Financial Instruments, Not Separately Reported, Mortgage-Related Securities Available-for-sale or Held-for-investment	437
Non-Agency [Member] \| Repurchase Agreements [Member]
Offsetting Assets and Liabilities [Line Items]
Pledged Financial Instruments, Not Separately Reported, Mortgage-Related Securities Available-for-sale or Held-for-investment	437
Non-Agency [Member] \| Derivative [Member]
Offsetting Assets and Liabilities [Line Items]
Pledged Financial Instruments, Not Separately Reported, Mortgage-Related Securities Available-for-sale or Held-for-investment	0
US Treasury Securities [Member]
Offsetting Assets and Liabilities [Line Items]
Pledged Financial Instruments, Not Separately Reported, Mortgage-Related Securities Available-for-sale or Held-for-investment	3,721	481
US Treasury Securities [Member] \| Under Prime Broker Agreements [Member]
Offsetting Assets and Liabilities [Line Items]
Pledged Financial Instruments, Not Separately Reported, Mortgage-Related Securities Available-for-sale or Held-for-investment	0
US Treasury Securities [Member] \| Repurchase Agreements [Member]
Offsetting Assets and Liabilities [Line Items]
Pledged Financial Instruments, Not Separately Reported, Mortgage-Related Securities Available-for-sale or Held-for-investment	3,514	453
US Treasury Securities [Member] \| Derivative [Member]
Offsetting Assets and Liabilities [Line Items]
Pledged Financial Instruments, Not Separately Reported, Mortgage-Related Securities Available-for-sale or Held-for-investment	207	0
Derivative [Member]
Offsetting Assets and Liabilities [Line Items]
Accrued interest on pledged securities	1
Repurchase Agreements [Member]
Offsetting Assets and Liabilities [Line Items]
Restricted cash	1,040	111
Under Prime Broker Agreements [Member]
Offsetting Assets and Liabilities [Line Items]
Restricted cash	0	0
Excluding Cash Received [Member]
Offsetting Assets and Liabilities [Line Items]
Restricted cash	$ 1,978	$ 451

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