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Available-For-Sale And Held-To-Maturity Securities (Narrative) (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2020 USD ($) security | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($)
Other Than Temporary Impairment Credit Losses Recognized In Earnings [Line Items] |  |  |
Proceeds from sale of available-for-sale securities | $ 491.9 | $ 641.8 | $ 372.4
Net realized gains (loss) resulting from sale of available-for-sale securities | (0.5) | (0.3) | (3.9)
Transfers from held to maturity to available for sale securities reclassification fair value |  | 1,100.0 |
Other comprehensive income (loss), transfers from held-to-maturity to available-for-sale securities, net of tax |  | 17.9 |
Transfers from held to maturity to available for sale securities reclassification fair value | 1,000.0 |  |
Unrealized gains (losses) recorded in accumulated other comprehensive loss | 33.1 | 42.4 | $ (36.3)
Financial instruments pledged as collateral | $ 368.4 | 801.5 |
Number of available for sale securities whose amortized costs exceeded their fair values | security | 40 |  |
Available-for-sale Securities, Continuous | $ 5.6 |  |
Available-for-sale Securities, Continuous Unrealized | 3.4 |  |
Available-for-sale Securities, Continuous | $ 524.1 |  |
Percentage of available-for-sale portfolio | 23.50% |  |
Pledged [Member] |  |  |
Other Than Temporary Impairment Credit Losses Recognized In Earnings [Line Items] |  |  |
Trading securities pledged | $ 1,400.0 | $ 816.1 |
Other Assets [Member] |  |  |
Other Than Temporary Impairment Credit Losses Recognized In Earnings [Line Items] |  |  |
Accrued interest receivable | $ 26.0 |  |

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