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Investment in Securities (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Dec. 31, 2020
Securities, Available-for-sale [Line Items]      
Debt Securities, Available-for-sale, Allowance for Credit Loss $ 19.7   $ 24.9
Debt Securities, Available-for-sale, Purchased with Credit Deterioration, Allowance for Credit Loss at Acquisition Date 0.6   2.6
Available-for-sale securities Realizes Losses - Write offs (1.2)    
Catch-up premium amortization adjustment (0.1) $ (1.1)  
Residential mortgage loans | Loans held-for-investment [Member]      
Securities, Available-for-sale [Line Items]      
Financing Receivable, Allowance for Credit Loss $ 1.6   $ 2.2