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Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Millions | Dec. 31, 2019 | Jan. 01, 2019 | Dec. 31, 2018
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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 | $ 677.3 |  |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Amortized Cost | 1,486.2 |  |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Amortized Cost | 328.0 |  |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Amortized Cost | 454.0 |  |
Amortized Cost | 2,945.5 |  | $ 2,305.2
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 | 661.4 |  |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value | 1,470.9 |  |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value | 428.2 |  |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value | 399.5 |  |
Estimated Fair Value | 2,960.0 |  | 2,270.7
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 | 38.6 |  |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost | 29.7 |  |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost | 21.3 |  |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Amortized Cost | 2.7 |  |
Amortized Cost | 92.3 | $ 281.1 | 406.8
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 | 38.6 |  |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value | 30.1 |  |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value | 23.1 |  |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value | 2.7 |  |
Estimated Fair Value | 94.5 | $ 275.1 | 400.7
Available-for-sale Securities and Held-to-maturity Securities |  |  |
Securities sold under agreements to repurchase | 2,132.0 |  | 1,943.1
Pledged assets separately reported, securities pledged for repurchase agreements, at fair value | 2,144.9 |  | 1,908.4
Mortgage Backed Securities, Variable Rate |  |  |
Available-for-sale Securities and Held-to-maturity Securities |  |  |
Available-for-sale securities, amortized cost basis | $ 298.1 |  | $ 219.5

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