Investment Securities (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2021	Jun. 30, 2020
Schedule of Investments [Line Items]
Net unsettled investment purchases	$ 300	$ 300	$ 300
Provision for credit losses on investment securities	0	26
Investment securities, allowance for credit loss	108	108
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	32,300	32,300
Fair value of debt securities of a single issuer that exceeds 10 percent of shareholders equity	33,200	33,200
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	22,500	22,500
Fair value of debt securities of a single issuer that exceeds 10 percent of shareholders equity	$ 22,600	$ 22,600