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Investment Securities (Tables) | 9 Months Ended
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Sep. 30, 2024
Investments, Debt and Equity Securities [Abstract] |
Amortized Cost and Approximate Fair Values of Investment Securities | The amortized cost and the approximate fair values of investment securities are summarized as follows:
September 30, 2024 | Amortized Cost |  |  | Gross Unrealized Gains | Gross Unrealized Losses | Estimated Fair Value
Available-for-Sale: |  |  |  |  |  |
U.S. Treasury notes | $ | 250.2 |  |  |  | $ | — |  | $ | (19.2) |  | $ | 231.0 |
State, county, and municipal securities | 252.8 |  |  |  | — |  | (31.9) |  | 220.9 |
Obligations of U.S. government agencies | 171.9 |  |  |  | — |  | (6.5) |  | 165.4 |
U.S. agency commercial mortgage-backed securities | 1,164.1 |  |  |  | 1.4 |  | (60.2) |  | 1,105.3 |
U.S. agency residential mortgage-backed securities | 1,354.2 |  |  |  | 2.4 |  | (94.2) |  | 1,262.4 |
Collateralized mortgage obligations | 1,202.1 |  |  |  | 2.2 |  | (87.7) |  | 1,116.6 |
Private mortgage-backed securities | 223.3 |  |  |  | — |  | (23.7) |  | 199.6 |
Collateralized loan obligation | 987.4 |  |  |  | 0.6 |  | — |  | 988.0 |
Corporate securities | 260.0 |  |  |  | — |  | (14.9) |  | 245.1 |
Total | $ | 5,866.0 |  |  |  | $ | 6.6 |  | $ | (338.3) |  | $ | 5,534.3 |
September 30, 2024 | Amortized Cost 1 | Allowance for Credit Losses | Net Amortized Cost | Gross Unrealized Gains | Gross Unrealized Losses | Estimated Fair Value
Held-to-Maturity: |  |  |  |  |  |
U.S. Treasury notes | $ | 99.6 |  | $ | — |  | $ | 99.6 |  | $ | — |  | $ | (0.4) |  | $ | 99.2 |
State, county, and municipal securities | 176.7 |  | — |  | 176.7 |  | 0.3 |  | (20.6) |  | 156.4 |
Obligations of U.S. government agencies | 356.4 |  | — |  | 356.4 |  | — |  | (32.0) |  | 324.4 |
U.S. agency commercial mortgage-backed securities | 493.0 |  | — |  | 493.0 |  | — |  | (41.8) |  | 451.2 |
U.S. agency residential mortgage-backed securities | 1,119.5 |  | — |  | 1,119.5 |  | 0.5 |  | (106.2) |  | 1,013.8 |
Collateralized mortgage obligations | 439.8 |  | — |  | 439.8 |  | 0.8 |  | (57.8) |  | 382.8 |
Corporate securities | 57.0 |  | (0.7) |  | 56.3 |  | — |  | (3.6) |  | 52.7 |
Total | $ | 2,742.0 |  | $ | (0.7) |  | $ | 2,741.3 |  | $ | 1.6 |  | $ | (262.4) |  | $ | 2,480.5 |
(1) Amortized cost presented above includes $8.6 million of unamortized gains in U.S. agency residential and commercial mortgage-backed securities and collateralized mortgage obligations related to the 2021 transfer of securities from available-for-sale to held-to-maturity, and $16.5 million of unamortized losses in state, county, and municipal, obligations of U.S. government agencies and collateralized loan obligations related to the 2022 transfer of securities from available-for-sale to held-to-maturity.
December 31, 2023 | Amortized Cost |  |  | Gross Unrealized Gains | Gross Unrealized Losses | Estimated Fair Value
Available-for-Sale: |  |  |  |  |  |
U.S. Treasury notes | $ | 250.2 |  |  |  | $ | — |  | $ | (25.5) |  | $ | 224.7 |
State, county, and municipal securities | 256.7 |  |  |  | — |  | (36.9) |  | 219.8 |
Obligations of U.S. government agencies | 179.4 |  |  |  | — |  | (10.9) |  | 168.5 |
U.S. agency commercial mortgage-backed securities | 1,192.7 |  |  |  | 0.6 |  | (87.7) |  | 1,105.6 |
U.S. agency residential mortgage-backed securities | 1,496.3 |  |  |  | 1.2 |  | (130.6) |  | 1,366.9 |
Collateralized mortgage obligations | 1,308.5 |  |  |  | 1.3 |  | (120.3) |  | 1,189.5 |
Private mortgage-backed securities | 241.3 |  |  |  | — |  | (30.9) |  | 210.4 |
Collateralized loan obligation | 1,121.9 |  |  |  | 0.1 |  | (2.3) |  | 1,119.7 |
Corporate securities | 260.8 |  |  |  | — |  | (24.4) |  | 236.4 |
Total | $ | 6,307.8 |  |  |  | $ | 3.2 |  | $ | (469.5) |  | $ | 5,841.5 |
December 31, 2023 | Amortized Cost 1 | Allowance for Credit Losses | Net Amortized Cost | Gross Unrealized Gains | Gross Unrealized Losses | Estimated Fair Value |
Held-to-Maturity: |  |  |  |  |  |  |
U.S. Treasury notes | $ | 399.0 |  | $ | — |  | $ | 399.0 |  | $ | — |  | $ | (2.8) |  | $ | 396.2 |  |
State, county, and municipal securities | 179.2 |  | — |  | 179.2 |  | 0.2 |  | (24.2) |  | 155.2 |  |
Obligations of U.S. government agencies | 354.5 |  | — |  | 354.5 |  | — |  | (42.4) |  | 312.1 |  |
U.S. agency commercial mortgage-backed securities | 510.5 |  | — |  | 510.5 |  | — |  | (52.9) |  | 457.6 |  |
U.S. agency residential mortgage-backed securities | 1,232.6 |  | — |  | 1,232.6 |  | — |  | (137.0) |  | 1,095.6 |  |
Collateralized mortgage obligations | 475.9 |  | — |  | 475.9 |  | 0.2 |  | (69.0) |  | 407.1 |  |
Corporate securities | 57.0 |  | (0.8) |  | 56.2 |  | — |  | (6.0) |  | 50.2 |  |
Total | $ | 3,208.7 |  | $ | (0.8) |  | $ | 3,207.9 |  | $ | 0.4 |  | $ | (334.3) |  | $ | 2,874.0 |  |
(1) Amortized cost presented above includes $10.7 million of unamortized gains and $18.1 million of unamortized losses related to the 2021 and 2022 transfer of securities from available-for-sale to held-to-maturity. |
Gross Unrealized Losses and Fair Values of Investment Securities | The following tables show the gross unrealized losses and fair values of available-for-sale investment securities and the length of time individual investment securities have been in an unrealized loss position as of September 30, 2024 and December 31, 2023.
| Less than 12 Months |  | 12 Months or More |  | Total
September 30, 2024 | Fair Value | Gross Unrealized Losses |  | Fair Value | Gross Unrealized Losses |  | Fair Value | Gross Unrealized Losses
Available-for-Sale: |  |  |  |  |  |  |  |
U.S. Treasury notes | $ | — |  | $ | — |  |  | $ | 231.0 |  | $ | (19.2) |  |  | $ | 231.0 |  | $ | (19.2) |
State, county, and municipal securities | — |  | — |  |  | 218.8 |  | (31.9) |  |  | 218.8 |  | (31.9) |
Obligations of U.S. government agencies | — |  | — |  |  | 165.1 |  | (6.5) |  |  | 165.1 |  | (6.5) |
U.S. agency commercial mortgage-backed securities | 0.1 |  | — |  |  | 1,048.8 |  | (60.2) |  |  | 1,048.9 |  | (60.2) |
U.S. agency residential mortgage-backed securities | — |  | — |  |  | 1,167.0 |  | (94.2) |  |  | 1,167.0 |  | (94.2) |
Collateralized mortgage obligations | 3.2 |  | — |  |  | 1,048.5 |  | (87.7) |  |  | 1,051.7 |  | (87.7) |
Private mortgage-backed securities | — |  | — |  |  | 199.5 |  | (23.7) |  |  | 199.5 |  | (23.7) |
Corporate securities | — |  | — |  |  | 217.0 |  | (14.9) |  |  | 217.0 |  | (14.9) |
Total | $ | 3.3 |  | $ | — |  |  | $ | 4,295.7 |  | $ | (338.3) |  |  | $ | 4,299.0 |  | $ | (338.3) |
| Less than 12 Months |  | 12 Months or More |  | Total
December 31, 2023 | Fair Value | Gross Unrealized Losses |  | Fair Value | Gross Unrealized Losses |  | Fair Value | Gross Unrealized Losses
Available-for-Sale: |  |  |  |  |  |  |  |
U.S. Treasury notes | $ | — |  | $ | — |  |  | $ | 224.7 |  | $ | (25.5) |  |  | $ | 224.7 |  | $ | (25.5) |
State, county, and municipal securities | — |  | — |  |  | 217.1 |  | (36.9) |  |  | 217.1 |  | (36.9) |
Obligations of U.S. government agencies | — |  | — |  |  | 168.0 |  | (10.9) |  |  | 168.0 |  | (10.9) |
U.S. agency commercial mortgage-backed securities | 0.2 |  | — |  |  | 1,083.1 |  | (87.7) |  |  | 1,083.3 |  | (87.7) |
U.S. agency residential mortgage-backed securities | 0.7 |  | — |  |  | 1,287.5 |  | (130.6) |  |  | 1,288.2 |  | (130.6) |
Collateralized mortgage obligations | — |  | — |  |  | 1,142.4 |  | (120.3) |  |  | 1,142.4 |  | (120.3) |
Private mortgage-backed securities | — |  | — |  |  | 210.5 |  | (30.9) |  |  | 210.5 |  | (30.9) |
Collateralized loan obligation | — |  | — |  |  | 935.7 |  | (2.3) |  |  | 935.7 |  | (2.3) |
Corporate securities | — |  | — |  |  | 236.5 |  | (24.4) |  |  | 236.5 |  | (24.4) |
Total | $ | 0.9 |  | $ | — |  |  | $ | 5,505.5 |  | $ | (469.5) |  |  | $ | 5,506.4 |  | $ | (469.5) |
Debt Securities, Held-to-maturity, Credit Quality Indicator | The following table summarizes the credit quality indicators of held-to-maturity securities at amortized cost for the periods indicated:
September 30, 2024 | AAA | AA | A | BBB | BB | Not Rated | Total
U.S. Treasury notes | $ | 99.6 |  | $ | — |  | $ | — |  | $ | — |  | $ | — |  | $ | — |  | $ | 99.6 |
State, county, and municipal securities | 85.7 |  | 74.2 |  | 10.1 |  | — |  | — |  | 6.7 |  | 176.7 |
Obligations of U.S. government agencies | 356.4 |  | — |  | — |  | — |  | — |  | — |  | 356.4 |
U.S. agency commercial mortgage-backed securities |  |  |  |  |  |  |
FNMA/FHLMC | 349.7 |  | — |  | — |  | — |  | — |  | — |  | 349.7 |
GNMA | 143.3 |  | — |  | — |  | — |  | — |  | — |  | 143.3 |
U.S. agency residential mortgage-backed securities |  |  |  |  |  |  |
FNMA/FHLMC | 1,080.7 |  | — |  | — |  | — |  | — |  | — |  | 1,080.7 |
GNMA | 38.8 |  | — |  | — |  | — |  | — |  | — |  | 38.8 |
Collateralized mortgage obligations |  |  |  |  |  |  |
FNMA/FHLMC | 308.5 |  | — |  | — |  | — |  | — |  | — |  | 308.5 |
GNMA | 131.3 |  | — |  | — |  | — |  | — |  | — |  | 131.3 |
Corporate securities | — |  | — |  | — |  | 47.0 |  | 5.0 |  | 5.0 |  | 57.0 |
Total | $ | 2,594.0 |  | $ | 74.2 |  | $ | 10.1 |  | $ | 47.0 |  | $ | 5.0 |  | $ | 11.7 |  | $ | 2,742.0 |
December 31, 2023 | AAA | AA | A | BBB | BB | Not Rated | Total
U.S. Treasury notes | $ | 399.0 |  | $ | — |  | $ | — |  | $ | — |  | $ | — |  | $ | — |  | $ | 399.0 |
State, county, and municipal securities | 69.0 |  | 92.3 |  | 10.6 |  | — |  | — |  | 7.3 |  | 179.2 |
Obligations of U.S. government agencies | 354.5 |  | — |  | — |  | — |  | — |  | — |  | 354.5 |
U.S. agency commercial mortgage-backed securities |  |  |  |  |  |  |
FNMA/FHLMC | 359.7 |  | — |  | — |  | — |  | — |  | — |  | 359.7 |
GNMA | 150.8 |  | — |  | — |  | — |  | — |  | — |  | 150.8 |
U.S. agency residential mortgage-backed securities |  |  |  |  |  |  |
FNMA/FHLMC | 1,189.8 |  | — |  | — |  | — |  | — |  | — |  | 1,189.8 |
GNMA | 42.8 |  | — |  | — |  | — |  | — |  | — |  | 42.8 |
Collateralized mortgage obligations |  |  |  |  |  |  |
FNMA/FHLMC | 334.1 |  | — |  | — |  | — |  | — |  | — |  | 334.1 |
GNMA | 141.8 |  | — |  | — |  | — |  | — |  | — |  | 141.8 |
Corporate securities | — |  | — |  | — |  | 47.0 |  | 5.0 |  | 5.0 |  | 57.0 |
Total | $ | 3,041.5 |  | $ | 92.3 |  | $ | 10.6 |  | $ | 47.0 |  | $ | 5.0 |  | $ | 12.3 |  | $ | 3,208.7 |
Maturities of Investment Securities |
| Available-for-Sale |  | Held-to-Maturity
September 30, 2024 | Amortized Cost | Estimated Fair Value |  | Amortized Cost | Estimated Fair Value
Within one year | $ | 88.7 |  | $ | 87.5 |  |  | $ | 101.1 |  | $ | 100.6 |
After one year but within five years | 1,326.9 |  | 1,265.9 |  |  | 416.0 |  | 396.8 |
After five years but within ten years | 1,166.4 |  | 1,095.1 |  |  | 485.5 |  | 437.5 |
After ten years | 3,284.0 |  | 3,085.8 |  |  | 1,739.4 |  | 1,545.6 |
Total | $ | 5,866.0 |  | $ | 5,534.3 |  |  | $ | 2,742.0 |  | $ | 2,480.5 |

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