Pledged Assets (Securities Pledged as Collateral) (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Offsetting Assets and Liabilities [Line Items]
Retained Interests in Consolidated VIE's Pledged as Collateral Under Repo	$ 111	$ 144
Security Owned and Pledged as Collateral, Fair Value	72,029	92,547
Agency Securities Pledged As Collateral Amortized Cost	68,824	90,887
Agency Securities Pledged As Collateral Accrued Interest	203	267
Maturity Less than 30 Days [Member]
Offsetting Assets and Liabilities [Line Items]
Security Owned and Pledged as Collateral, Fair Value	46,347	56,990
Agency Securities Pledged As Collateral Amortized Cost	44,231	55,951
Agency Securities Pledged As Collateral Accrued Interest	131	167
Maturity 31 To 59 Days [Member]
Offsetting Assets and Liabilities [Line Items]
Security Owned and Pledged as Collateral, Fair Value	7,659	14,410
Agency Securities Pledged As Collateral Amortized Cost	7,308	14,114
Agency Securities Pledged As Collateral Accrued Interest	22	42
Maturity 60 To 90 Days [Member]
Offsetting Assets and Liabilities [Line Items]
Security Owned and Pledged as Collateral, Fair Value	2,953	7,637
Agency Securities Pledged As Collateral Amortized Cost	2,838	7,536
Agency Securities Pledged As Collateral Accrued Interest	8	20
Maturity over 90 days [Member]
Offsetting Assets and Liabilities [Line Items]
Security Owned and Pledged as Collateral, Fair Value	15,070	13,510
Agency Securities Pledged As Collateral Amortized Cost	14,447	13,286
Agency Securities Pledged As Collateral Accrued Interest	42	38
Repurchase Agreements [Member]
Offsetting Assets and Liabilities [Line Items]
Security Owned and Pledged as Collateral, Fair Value	70,357	92,142
Derivative [Member]
Offsetting Assets and Liabilities [Line Items]
Security Owned and Pledged as Collateral, Fair Value	0	404
Credit Risk Transfer Securities [Member]
Offsetting Assets and Liabilities [Line Items]
Security Owned and Pledged as Collateral, Fair Value	490	309
Credit Risk Transfer Securities [Member] | Repurchase Agreements [Member]
Offsetting Assets and Liabilities [Line Items]
Security Owned and Pledged as Collateral, Fair Value	490	309
Credit Risk Transfer Securities [Member] | Derivative [Member]
Offsetting Assets and Liabilities [Line Items]
Security Owned and Pledged as Collateral, Fair Value	0	0
US Treasury Securities [Member]
Offsetting Assets and Liabilities [Line Items]
Security Owned and Pledged as Collateral, Fair Value	1,136	481
Securities Received as Collateral, Amount Repledged and Sold	0	357
US Treasury Securities [Member] | Repurchase Agreements [Member]
Offsetting Assets and Liabilities [Line Items]
Security Owned and Pledged as Collateral, Fair Value	668	453
US Treasury Securities [Member] | Derivative [Member]
Offsetting Assets and Liabilities [Line Items]
Security Owned and Pledged as Collateral, Fair Value	468	$ 0
Non-Agency [Member]
Offsetting Assets and Liabilities [Line Items]
Security Owned and Pledged as Collateral, Fair Value	514
Non-Agency [Member] | Repurchase Agreements [Member]
Offsetting Assets and Liabilities [Line Items]
Security Owned and Pledged as Collateral, Fair Value	514
Non-Agency [Member] | Derivative [Member]
Offsetting Assets and Liabilities [Line Items]
Security Owned and Pledged as Collateral, Fair Value	$ 0