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Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
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Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity |  |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Amortized Cost | $ 687.6 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Amortized Cost | 1,587.7 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Amortized Cost | 736.2 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Amortized Cost | 921.2 |
Amortized Cost | 3,932.7 | $ 2,945.5
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity |  |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value | 991.4 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value | 1,362.6 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value | 996.3 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value | 658.4 |
Estimated Fair Value | 4,008.7 | 2,960.0
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount |  |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost | 8.3 |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost | 27.8 |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost | 14.1 |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Amortized Cost | 1.4 |
Amortized Cost | 51.6 | 92.3
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity |  |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value | 8.4 |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value | 29.0 |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value | 16.2 |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value | 1.4 |
Estimated Fair Value | 55.0 | 94.5
Available-for-sale Securities and Held-to-maturity Securities |  |
Securities sold under agreements to repurchase | 2,323.0 | 2,132.0
Pledged assets separately reported, securities pledged for repurchase agreements, at fair value | 2,383.6 | 2,144.9
Mortgage Backed Securities, Variable Rate |  |
Available-for-sale Securities and Held-to-maturity Securities |  |
Available-for-sale securities, amortized cost basis | $ 220.7 | $ 298.1

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