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Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) $ in Millions | Dec. 31, 2023 USD ($) Investments | Dec. 31, 2022 USD ($) Investments
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Investments, Debt and Equity Securities [Abstract] |  |
Debt Securities, Held-to-maturity | $ 3,208.7 | $ 3,453.7
Available-for-Sale and Held-to-Maturity: |  |
Investment securities in an unrealized loss position (number of securities) | Investments | 995 | 1,222
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | $ 0.9 | $ 5,071.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 5,505.5 | 1,545.6
Debt Securities, Available-for-sale, Unrealized Loss Position | 5,506.4 | 6,617.5
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | (469.5) | (629.7)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0.0 | (346.6)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (469.5) | (283.1)
US Government Agencies Debt Securities [Member] |  |
Investments, Debt and Equity Securities [Abstract] |  |
Debt Securities, Held-to-maturity | 354.5 | 351.7
Available-for-Sale and Held-to-Maturity: |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 0.0 | 152.2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 168.0 | 46.1
Debt Securities, Available-for-sale, Unrealized Loss Position | 168.0 | 198.3
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | (10.9) | (17.3)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0.0 | (10.1)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (10.9) | (7.2)
Corporate Debt Securities [Member] |  |
Investments, Debt and Equity Securities [Abstract] |  |
Debt Securities, Held-to-maturity | 57.0 | 80.1
Available-for-Sale and Held-to-Maturity: |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 0.0 | 130.6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 236.5 | 110.8
Debt Securities, Available-for-sale, Unrealized Loss Position | 236.5 | 241.4
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | (24.4) | (30.8)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0.0 | (8.3)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (24.4) | (22.5)
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 | 0.0 | 133.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 210.5 | 94.9
Debt Securities, Available-for-sale, Unrealized Loss Position | 210.5 | 228.0
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | (30.9) | (36.9)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0.0 | (19.1)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (30.9) | (17.8)
US Treasury notes |  |
Investments, Debt and Equity Securities [Abstract] |  |
Debt Securities, Held-to-maturity | 399.0 | 396.6
Available-for-Sale and Held-to-Maturity: |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 0.0 | 168.8
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 224.7 | 170.4
Debt Securities, Available-for-sale, Unrealized Loss Position | 224.7 | 339.2
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | (25.5) | (34.5)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0.0 | (4.7)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (25.5) | (29.8)
State, county and municipal securities |  |
Investments, Debt and Equity Securities [Abstract] |  |
Debt Securities, Held-to-maturity | 179.2 | 181.2
Available-for-Sale and Held-to-Maturity: |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 0.0 | 104.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 217.1 | 146.1
Debt Securities, Available-for-sale, Unrealized Loss Position | 217.1 | 251.0
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | (36.9) | (50.6)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0.0 | (9.0)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (36.9) | (41.6)
Collateralized Loan Obligations |  |
Available-for-Sale and Held-to-Maturity: |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 0.0 | 1,082.6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 935.7 | 28.9
Debt Securities, Available-for-sale, Unrealized Loss Position | 935.7 | 1,111.5
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | (2.3) | (33.6)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0.0 | (33.1)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (2.3) | (0.5)
Commercial Mortgage Backed Securities |  |
Investments, Debt and Equity Securities [Abstract] |  |
Debt Securities, Held-to-maturity | 510.5 | 526.8
Available-for-Sale and Held-to-Maturity: |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 0.2 | 1,069.8
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 1,083.1 | 270.8
Debt Securities, Available-for-sale, Unrealized Loss Position | 1,083.3 | 1,340.6
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | (87.7) | (116.9)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0.0 | (59.9)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (87.7) | (57.0)
Residential Mortgage-Backed Securities |  |
Investments, Debt and Equity Securities [Abstract] |  |
Debt Securities, Held-to-maturity | 1,232.6 | 1,391.5
Available-for-Sale and Held-to-Maturity: |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 0.7 | 1,213.5
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 1,287.5 | 344.6
Debt Securities, Available-for-sale, Unrealized Loss Position | 1,288.2 | 1,558.1
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | (130.6) | (167.7)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0.0 | (112.3)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (130.6) | (55.4)
Collateralized Mortgage Obligations |  |
Investments, Debt and Equity Securities [Abstract] |  |
Debt Securities, Held-to-maturity | 475.9 | 525.8
Available-for-Sale and Held-to-Maturity: |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 0.0 | 1,016.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 1,142.4 | 333.0
Debt Securities, Available-for-sale, Unrealized Loss Position | 1,142.4 | 1,349.4
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | (120.3) | (141.4)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0.0 | (90.1)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | $ (120.3) | $ (51.3)

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