Investment Securities (Accrued Interest Receivable Excluded from Other Values - Narrative) (Details) - Accrued Interest Receivable - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Accrued interest receivable	$ 30.9	$ 30.7
Available-for-sale Securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Accrued interest receivable	7.1	7.5
Held-to-maturity Securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Accrued interest receivable	$ 8.9	$ 8.1