Investment Securities (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2020	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019	Sep. 30, 2019
Schedule of Investments [Line Items]
Investment securities, allowance for credit loss	$ 71	$ 71
Provision for credit losses on investment securities	39	69
Available for sale debt securities, amortized cost	86,344	86,344			$ 67,790
Available-for-sale securities, fair value	89,747	89,747			69,163
Investment securities, net unsettled investment purchases	700	700				$ 300
Held-to-maturity securities, amortized cost	1,438	1,438			17,661
Held-to-maturity securities, fair value	1,614	1,614			$ 18,044
Federal National Mortgage Association Certificates and Obligations (FNMA) [Member]
Schedule of Investments [Line Items]
Debt securities amortized cost amount of debt securities exceeds 10 percent of shareholders equity	36,900	36,900
Fair value of debt securities of a single issuer that exceeeds 10 percent of shareholders equity	38,300	38,300
Federal Home Loan Mortgage Corporation (FHLMC) Insured Loans [Member]
Schedule of Investments [Line Items]
Debt securities amortized cost amount of debt securities exceeds 10 percent of shareholders equity	9,800	9,800
Fair value of debt securities of a single issuer that exceeeds 10 percent of shareholders equity	$ 10,100	$ 10,100
Cumulative Effect, Period of Adoption, Adjustment | Accounting Standards Update 2019-04
Schedule of Investments [Line Items]
Available for sale debt securities, amortized cost				$ 16,200
Available-for-sale securities, fair value				16,500
Held-to-maturity securities, amortized cost				(16,200)
Held-to-maturity securities, fair value				$ (16,500)
Cumulative Effect, Period of Adoption, Adjustment | Accounting Standards Update 2020-04
Schedule of Investments [Line Items]
Available for sale debt securities, amortized cost			$ 49
Available-for-sale securities, fair value			48
Held-to-maturity securities, amortized cost			(49)
Held-to-maturity securities, fair value			$ (48)