Investment Securities (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Document Period End Date	Dec. 31, 2018
Mortgage-backed Securities Available-for-sale, Fair Value Disclosure	$ 84,287	$ 57,080
Weighted average expected constant prepayment rate	8.00%	8.00%
Variable Interest Entity, Consolidated, Carrying Amount, Assets	$ (436)	$ (662)
Debt, at fair value	(275)	(357)
Agency Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Unamortized premium balance	2,900	2,700
TBA securities Fifteen Year and Thirty Year Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Net long TBA position, at fair value	7,300	15,700
TBA, net carrying value	$ 70	$ 3
Credit Risk Transfer Securities [Member] \| Minimum [Member]
Debt Securities, Available-for-sale [Line Items]
Weighted Average Coupon Rate	3.90%	3.90%
Underlying Collateral Coupon	3.80%	3.60%
Credit Risk Transfer Securities [Member] \| Maximum [Member]
Debt Securities, Available-for-sale [Line Items]
Weighted Average Coupon Rate	9.50%	8.50%
Underlying Collateral Coupon	4.80%	4.40%

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