Investment Securities Available for Sale (Narrative) (Details)	3 Months Ended			9 Months Ended
	Sep. 30, 2020 USD ($) securities	Jun. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) securities	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) securities
Financing Receivable, Impaired [Line Items]
Investment securities gains, net	$ (1,550,000)	$ 69,400,000	$ (3,731,000)	$ 76,594,000	$ (5,502,000)
Gross losses	1,550,000	5,700,000	3,871,000	7,246,000	14,772,000
Gross realized gains on sales	$ 0		140,000	$ 83,840,000	9,270,000
Investment securities in a loss position for less than twelve months | securities	28			28		26
Investment securities in a loss position for twelve months or longer | securities	0			0		5
Investment securities gains, net	$ (1,550,000)	$ 69,400,000	$ (3,731,000)	$ 76,594,000	$ (5,502,000)
Debt Securities, Available-for-sale, Allowance for Credit Loss	0			0
Debt Securities, Available-for-sale, Realized Loss - Quarter Two Settlement	802,000
Collateral Pledged
Financing Receivable, Impaired [Line Items]
Pledged to secure deposits	$ 2,340,000,000			$ 2,340,000,000		$ 1,710,000,000