Investment Securities (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2021	Sep. 30, 2020
Schedule of Investments [Line Items]
Net unsettled investment purchases	$ 2,400	$ 2,400	$ 700
Investment securities, allowance for credit loss	133	133
Provision for credit losses on investment securities	25	51
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	33,100	33,100
Fair value of debt securities of a single issuer that exceeds 10 percent of shareholders equity	33,800	33,800
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	21,600	21,600
Fair value of debt securities of a single issuer that exceeds 10 percent of shareholders equity	$ 21,600	$ 21,600