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Investment Securities (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Schedule of Investments [Line Items]
Net unsettled investment purchases	$ 1,800	$ 400
Investment securities, allowance for credit loss	108
Provision for credit losses on investment securities	26
Federal National Mortgage Association Certificates and Obligations (FNMA)
Schedule of Investments [Line Items]
Debt securities amortized cost amount of debt securities exceeds 10 percent of shareholders equity	30,800
Fair value of debt securities of a single issuer that exceeeds 10 percent of shareholders equity	31,700
Federal Home Loan Mortgage Corporation (FHLMC) Insured Loans
Schedule of Investments [Line Items]
Debt securities amortized cost amount of debt securities exceeds 10 percent of shareholders equity	18,900
Fair value of debt securities of a single issuer that exceeeds 10 percent of shareholders equity	$ 18,800