Pledged Assets (Assets Pledged as Collateral) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Offsetting Assets and Liabilities [Line Items]
Pledged Financial Instruments, Not Separately Reported, Mortgage-Related Securities Available-for-sale or Held-for-investment	$ 79,182	$ 52,497
Document Period End Date	Dec. 31, 2018
Retained Interests in Consolidated VIE's Pledged as Collateral Under Repurchase Agreements	$ 163	182
Accrued interest on pledged securities	248	158
Restricted cash and cash equivalents	599	317
Total Fair Value of Securities Pledged and Accrued Interest	81,210
Total Fair Value Of Agency Securities Pledged And Accrued Interest		54,559
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	78,997	52,497
Derivative [Member]
Offsetting Assets and Liabilities [Line Items]
Pledged Financial Instruments, Not Separately Reported, Mortgage-Related Securities Available-for-sale or Held-for-investment	174	221
Accrued interest on pledged securities		1
Restricted cash and cash equivalents	522	281
Total Fair Value of Securities Pledged and Accrued Interest	697
Total Fair Value Of Agency Securities Pledged And Accrued Interest		575
Repurchase Agreements [Member]
Offsetting Assets and Liabilities [Line Items]
Accrued interest on pledged securities	246	153
Total Fair Value of Securities Pledged and Accrued Interest	79,943
Total Fair Value Of Agency Securities Pledged And Accrued Interest		52,798
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	436	662
Accrued interest on pledged securities	1	2
Total Fair Value of Securities Pledged and Accrued Interest	437
Total Fair Value Of Agency Securities Pledged And Accrued Interest		664
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	133	519
Accrued interest on pledged securities	0	2
Total Fair Value of Securities Pledged and Accrued Interest	133
Total Fair Value Of Agency Securities Pledged And Accrued Interest		522
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	79,740	53,899
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	141	0
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	141
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
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	45
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	45
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	437	185
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	437	113
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	0	72
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 and cash equivalents	77	35
Under Prime Broker Agreements [Member]
Offsetting Assets and Liabilities [Line Items]
Restricted cash and cash equivalents	0	1
Excluding Cash Received [Member]
Offsetting Assets and Liabilities [Line Items]
Restricted cash and cash equivalents	$ 599	$ 317