Investment Securities (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Mortgage-backed Securities Available-for-sale, Fair Value Disclosure	$ 71,800	$ 100,400
Weighted average expected constant prepayment rate	14.50%	10.80%
Debt, at fair value	$ (214)	$ (228)
Agency Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Unamortized premium balance	2,600	3,100
TBA securities Fifteen Year and Thirty Year Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Net long TBA position, at fair value	21,200	7,400
TBA, net carrying value	$ 574	$ 25

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