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Investment Securities (Tables) | 12 Months Ended
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Dec. 31, 2023
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:
December 31, 2023 | Amortized Cost | Allowance for Credit Losses | Net Amortized Cost | Gross Unrealized Gains | Gross Unrealized Losses | Estimated Fair Value
Available-for-Sale: |  |  |  |  |  |
U.S. Treasury notes | $ | 250.2 |  | $ | — |  | $ | 250.2 |  | $ | — |  | $ | (25.5) |  | $ | 224.7 |
State, county, and municipal securities | 256.7 |  | — |  | 256.7 |  | — |  | (36.9) |  | 219.8 |
Obligations of U.S. government agencies | 179.4 |  | — |  | 179.4 |  | — |  | (10.9) |  | 168.5 |
U.S. agency commercial mortgage-backed securities | 1,192.7 |  | — |  | 1,192.7 |  | 0.6 |  | (87.7) |  | 1,105.6 |
U.S. agency residential mortgage-backed securities | 1,496.3 |  | — |  | 1,496.3 |  | 1.2 |  | (130.6) |  | 1,366.9 |
Collateralized mortgage obligations | 1,308.5 |  | — |  | 1,308.5 |  | 1.3 |  | (120.3) |  | 1,189.5 |
Private mortgage-backed securities | 241.3 |  | — |  | 241.3 |  | — |  | (30.9) |  | 210.4 |
Collateralized loan obligations | 1,121.9 |  | — |  | 1,121.9 |  | 0.1 |  | (2.3) |  | 1,119.7 |
Corporate securities | 260.8 |  | — |  | 260.8 |  | — |  | (24.4) |  | 236.4 |
Total | $ | 6,307.8 |  | $ | — |  | $ | 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 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 $18.1 million of unamortized losses in state, county, and municipal, obligations of U.S. government agencies and corporate securities related to the 2022 transfer of securities from available-for-sale to held-to-maturity.
December 31, 2022 | Amortized Cost | Allowance for Credit Losses | Net Amortized Cost | Gross Unrealized Gains | Gross Unrealized Losses | Estimated Fair Value
Available-for-Sale |  |  |  |  |  |
U.S. Treasury notes | $ | 675.1 |  | $ | — |  | $ | 675.1 |  | $ | 2.1 |  | $ | (34.5) |  | $ | 642.7 |
State, county, and municipal securities | 314.3 |  | — |  | 314.3 |  | — |  | (50.6) |  | 263.7 |
Obligations of U.S. government agencies | 216.2 |  | — |  | 216.2 |  | — |  | (17.3) |  | 198.9 |
U.S. agency commercial mortgage-backed securities | 1,468.0 |  | — |  | 1,468.0 |  | 0.2 |  | (117.0) |  | 1,351.2 |
U.S. agency residential mortgage-backed securities | 1,726.0 |  | — |  | 1,726.0 |  | — |  | (167.7) |  | 1,558.3 |
Collateralized mortgage obligations | 1,491.5 |  | — |  | 1,491.5 |  | — |  | (141.3) |  | 1,350.2 |
Private mortgage-backed securities | 264.9 |  | — |  | 264.9 |  | — |  | (36.9) |  | 228.0 |
Collateralized loan obligations | 1,145.2 |  | — |  | 1,145.2 |  | — |  | (33.6) |  | 1,111.6 |
Corporate securities | 272.3 |  | — |  | 272.3 |  | — |  | (30.8) |  | 241.5 |
Total | $ | 7,573.5 |  | $ | — |  | $ | 7,573.5 |  | $ | 2.3 |  | $ | (629.7) |  | $ | 6,946.1 |
December 31, 2022 | 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 | $ | 396.6 |  | $ | — |  | $ | 396.6 |  | $ | — |  | $ | (10.2) |  | $ | 386.4 |
State, county, and municipal securities | 181.2 |  | (0.1) |  | 181.1 |  | 0.2 |  | (31.6) |  | 149.7 |
Obligations of U.S. government agencies | 351.7 |  | — |  | 351.7 |  | — |  | (49.6) |  | 302.1 |
U.S. agency commercial mortgage-backed securities | 526.8 |  | — |  | 526.8 |  | — |  | (58.3) |  | 468.5 |
U.S. agency residential mortgage-backed securities | 1,391.5 |  | — |  | 1,391.5 |  | — |  | (166.6) |  | 1,224.9 |
Collateralized mortgage obligations | 525.8 |  | — |  | 525.8 |  | 0.2 |  | (76.7) |  | 449.3 |
Corporate securities | 80.1 |  | (1.8) |  | 78.3 |  | — |  | (7.0) |  | 71.3 |
Total | $ | 3,453.7 |  | $ | (1.9) |  | $ | 3,451.8 |  | $ | 0.4 |  | $ | (400.0) |  | $ | 3,052.2 |
(1) Amortized cost presented above includes $20.2 million of unamortized losses and of $14.2 million unamortized gains 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 December 31, 2023 and 2022.
| 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 obligations | — |  | — |  |  | 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) |
| Less than 12 Months |  | 12 Months or More |  | Total
December 31, 2022 | Fair Value | Gross Unrealized Losses |  | Fair Value | Gross Unrealized Losses |  | Fair Value | Gross Unrealized Losses
Available-for-Sale |  |  |  |  |  |  |  |
U.S. Treasury notes | $ | 168.8 |  | $ | (4.7) |  |  | $ | 170.4 |  | $ | (29.8) |  |  | $ | 339.2 |  | $ | (34.5) |
State, county, and municipal securities | 104.9 |  | (9.0) |  |  | 146.1 |  | (41.6) |  |  | 251.0 |  | (50.6) |
Obligations of U.S. government agencies | 152.2 |  | (10.1) |  |  | 46.1 |  | (7.2) |  |  | 198.3 |  | (17.3) |
U.S. agency commercial mortgage-backed securities | 1,069.8 |  | (59.9) |  |  | 270.8 |  | (57.0) |  |  | 1,340.6 |  | (116.9) |
U.S. agency residential mortgage-backed securities | 1,213.5 |  | (112.3) |  |  | 344.6 |  | (55.4) |  |  | 1,558.1 |  | (167.7) |
Collateralized mortgage obligations | 1,016.4 |  | (90.1) |  |  | 333.0 |  | (51.3) |  |  | 1,349.4 |  | (141.4) |
Private mortgage-backed securities | 133.1 |  | (19.1) |  |  | 94.9 |  | (17.8) |  |  | 228.0 |  | (36.9) |
Collateralized loan obligations | 1,082.6 |  | (33.1) |  |  | 28.9 |  | (0.5) |  |  | 1,111.5 |  | (33.6) |
Corporate securities | 130.6 |  | (8.3) |  |  | 110.8 |  | (22.5) |  |  | 241.4 |  | (30.8) |
Total | $ | 5,071.9 |  | $ | (346.6) |  |  | $ | 1,545.6 |  | $ | (283.1) |  |  | $ | 6,617.5 |  | $ | (629.7) |
Debt Securities, Held-to-maturity, Allowance for Credit Loss |
The following table presents the activity in the allowance for credit losses related to held-to-maturity investment securities:
Year Ended December 31, | 2023 |  | 2022 |  | 2021
Beginning balance | $ | 1.9 |  |  | $ | — |  |  | $ | — |
(Reduction of) provision for credit losses | (1.1) |  |  | 1.9 |  |  | — |
Ending balance | $ | 0.8 |  |  | $ | 1.9 |  |  | $ | — |
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:
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 |
December 31, 2022 | AAA | AA | A | BBB | BB | Not Rated | Total
U.S. Treasury notes | $ | 396.6 |  | $ | — |  | $ | — |  | $ | — |  | $ | — |  | $ | — |  | $ | 396.6 |
State, county, and municipal securities | 68.3 |  | 92.8 |  | 11.5 |  | — |  | — |  | 8.6 |  | 181.2 |
Obligations of U.S. government agencies | 351.7 |  | — |  | — |  | — |  | — |  | — |  | 351.7 |
U.S. agency commercial mortgage-backed securities |  |  |  |  |  |  |
FNMA/FHLMC | 364.9 |  | — |  | — |  | — |  | — |  | — |  | 364.9 |
GNMA | 161.9 |  | — |  | — |  | — |  | — |  | — |  | 161.9 |
U.S. agency residential mortgage-backed securities |  |  |  |  |  |  |
FNMA/FHLMC | 1,342.9 |  | — |  | — |  | — |  | — |  | — |  | 1,342.9 |
GNMA | 48.6 |  | — |  | — |  | — |  | — |  | — |  | 48.6 |
Collateralized mortgage obligations |  |  |  |  |  |  |
FNMA/FHLMC | 525.8 |  | — |  | — |  | — |  | — |  | — |  | 525.8 |
Corporate securities | — |  | — |  | — |  | 65.1 |  | 10.0 |  | 5.0 |  | 80.1 |
Total | $ | 3,260.7 |  | $ | 92.8 |  | $ | 11.5 |  | $ | 65.1 |  | $ | 10.0 |  | $ | 13.6 |  | $ | 3,453.7 |
Maturities of Investment Securities |
| Available-for-Sale |  | Held-to-Maturity
December 31, 2023 | Amortized Cost | Estimated Fair Value |  | Amortized Cost | Estimated Fair Value
Within one year | $ | 40.1 |  | $ | 39.4 |  |  | $ | 309.1 |  | $ | 307.5 |
After one year but within five years | 1,004.4 |  | 938.7 |  |  | 346.6 |  | 329.4 |
After five years but within ten years | 1,373.9 |  | 1,255.4 |  |  | 654.0 |  | 579.1 |
After ten years | 3,889.4 |  | 3,608.0 |  |  | 1,899.0 |  | 1,658.0 |
Total | $ | 6,307.8 |  | $ | 5,841.5 |  |  | $ | 3,208.7 |  | $ | 2,874.0 |

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- Definition For all investments in an unrealized loss position, including those for which other-than-temporary impairments have not been recognized in earnings (including investments for which a portion of an other-than-temporary impairment has been recognized in other comprehensive income), a tabular disclosure of the aggregate related fair value of investments with unrealized losses and the aggregate amount of unrealized losses (that is, the amount by which amortized cost basis exceeds fair value). + References Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-6 + Details
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