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Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) $ in Millions | Dec. 31, 2022 USD ($) Investments | Dec. 31, 2021 USD ($) Investments
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Investments, Debt and Equity Securities [Abstract] |  |
Debt Securities, Held-to-maturity | $ 3,453.7 | $ 1,687.6
Available-for-Sale and Held-to-Maturity: |  |
Investment securities in an unrealized loss position (number of securities) | Investments | 1,222 | 285
Less than 12 Months | $ 1,923.9 | $ 1,067.7
Less than 12 Months | (187.2) | (35.7)
12 Months or More | 1,075.2 | 0.0
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (212.8) | 0.0
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value | 2,999.1 | 1,067.7
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss | (400.0) | (35.7)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 5,071.9 | 2,760.3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 1,545.6 | 68.7
Debt Securities, Available-for-sale, Unrealized Loss Position | 6,617.5 | 2,829.0
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | (629.7) | (59.4)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (346.6) | (58.3)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (283.1) | (1.1)
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] |  |
Investments, Debt and Equity Securities [Abstract] |  |
Debt Securities, Held-to-maturity | 2,444.1 | 1,609.0
Available-for-Sale and Held-to-Maturity: |  |
Less than 12 Months | 1,349.4 | 1,038.7
Less than 12 Months | (156.1) | (35.3)
12 Months or More | 762.5 | 0.0
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (145.5) | 0.0
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value | 2,111.9 | 1,038.7
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss | (301.6) | (35.3)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 3,299.7 | 1,262.8
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 948.4 | 26.4
Debt Securities, Available-for-sale, Unrealized Loss Position | 4,248.1 | 1,289.2
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | (426.0) | (23.3)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (262.3) | (23.0)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (163.7) | (0.3)
US Government Agencies Debt Securities [Member] |  |
Investments, Debt and Equity Securities [Abstract] |  |
Debt Securities, Held-to-maturity | 351.7 |
Available-for-Sale and Held-to-Maturity: |  |
Less than 12 Months | 86.1 |
Less than 12 Months | (9.9) |
12 Months or More | 216.0 |
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (39.7) |
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value | 302.1 |
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss | (49.6) |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 152.2 | 297.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 46.1 | 16.4
Debt Securities, Available-for-sale, Unrealized Loss Position | 198.3 | 313.4
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | (17.3) | (9.2)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (10.1) | (8.9)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (7.2) | (0.3)
Corporate Debt Securities [Member] |  |
Investments, Debt and Equity Securities [Abstract] |  |
Debt Securities, Held-to-maturity | 80.1 | 11.0
Available-for-Sale and Held-to-Maturity: |  |
Less than 12 Months | 68.2 |
Less than 12 Months | (7.0) |
12 Months or More | 0.0 |
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0.0 |
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value | 68.2 |
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss | (7.0) |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 130.6 | 109.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 110.8 | 20.9
Debt Securities, Available-for-sale, Unrealized Loss Position | 241.4 | 130.8
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | (30.8) | (3.6)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (8.3) | (3.3)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (22.5) | (0.3)
Mortgage-backed Securities, Issued by Private Enterprises [Member] |  |
Available-for-Sale and Held-to-Maturity: |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 133.1 | 127.2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 94.9 | 0.0
Debt Securities, Available-for-sale, Unrealized Loss Position | 228.0 | 127.2
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | (36.9) | (1.1)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (19.1) | (1.1)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (17.8) | 0.0
US Treasury notes |  |
Investments, Debt and Equity Securities [Abstract] |  |
Debt Securities, Held-to-maturity | 396.6 |
Available-for-Sale and Held-to-Maturity: |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 168.8 | 684.7
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 170.4 | 0.0
Debt Securities, Available-for-sale, Unrealized Loss Position | 339.2 | 684.7
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | (34.5) | (12.9)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (4.7) | (12.9)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (29.8) | 0.0
State, county and municipal securities |  |
Investments, Debt and Equity Securities [Abstract] |  |
Debt Securities, Held-to-maturity | 181.2 | 67.6
Available-for-Sale and Held-to-Maturity: |  |
Less than 12 Months | 33.8 | 29.0
Less than 12 Months | (4.0) | (0.4)
12 Months or More | 96.7 | 0.0
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (27.6) | 0.0
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value | 130.5 | 29.0
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss | (31.6) | (0.4)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 104.9 | 278.7
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 146.1 | 5.0
Debt Securities, Available-for-sale, Unrealized Loss Position | 251.0 | 283.7
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | (50.6) | (9.3)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (9.0) | (9.1)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (41.6) | $ (0.2)
Collateralized Loan Obligations |  |
Available-for-Sale and Held-to-Maturity: |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 1,082.6 |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 28.9 |
Debt Securities, Available-for-sale, Unrealized Loss Position | 1,111.5 |
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | (33.6) |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (33.1) |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (0.5) |
US Treasury Notes Securities |  |
Available-for-Sale and Held-to-Maturity: |  |
Less than 12 Months | 386.4 |
Less than 12 Months | (10.2) |
12 Months or More | 0.0 |
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0.0 |
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value | 386.4 |
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss | $ (10.2) |

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