Securities (Narrative) (Details)	Sep. 30, 2020 USD ($) Security	Jan. 01, 2020 USD ($)	Dec. 31, 2019 USD ($)
Investments Debt And Equity Securities [Abstract]
Amortized cost of securities excluding accrued interest	$ 23,200,000		$ 23,900,000
Securities classified as trading	0		0
Securities pledged as collateral	3,200,000,000		$ 3,300,000,000
Allowance for credit loss	$ 0	$ 0
Securities that met the criteria of a credit loss event | Security	0
Number of securities with market values below their cost basis | Security	40