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Investment Securities (Tables)
9 Months Ended
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, 2024Amortized
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 securities252.8 — (31.9)220.9 
Obligations of U.S. government agencies171.9 — (6.5)165.4 
U.S. agency commercial mortgage-backed securities1,164.1 1.4 (60.2)1,105.3 
U.S. agency residential mortgage-backed securities1,354.2 2.4 (94.2)1,262.4 
Collateralized mortgage obligations1,202.1 2.2 (87.7)1,116.6 
Private mortgage-backed securities223.3 — (23.7)199.6 
Collateralized loan obligation987.4 0.6 — 988.0 
Corporate securities260.0 — (14.9)245.1 
Total$5,866.0 $6.6 $(338.3)$5,534.3 
September 30, 2024
Amortized
Cost1
Allowance for Credit LossesNet Amortized CostGross
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 securities176.7 — 176.7 0.3 (20.6)156.4 
Obligations of U.S. government agencies356.4 — 356.4 — (32.0)324.4 
U.S. agency commercial mortgage-backed securities493.0 — 493.0 — (41.8)451.2 
U.S. agency residential mortgage-backed securities1,119.5 — 1,119.5 0.5 (106.2)1,013.8 
Collateralized mortgage obligations439.8 — 439.8 0.8 (57.8)382.8 
Corporate securities57.0 (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, 2023Amortized
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 securities256.7 — (36.9)219.8 
Obligations of U.S. government agencies179.4 — (10.9)168.5 
U.S. agency commercial mortgage-backed securities1,192.7 0.6 (87.7)1,105.6 
U.S. agency residential mortgage-backed securities1,496.3 1.2 (130.6)1,366.9 
Collateralized mortgage obligations1,308.5 1.3 (120.3)1,189.5 
Private mortgage-backed securities241.3 — (30.9)210.4 
Collateralized loan obligation1,121.9 0.1 (2.3)1,119.7 
Corporate securities260.8 — (24.4)236.4 
Total$6,307.8 $3.2 $(469.5)$5,841.5 

December 31, 2023
Amortized
Cost1
Allowance for Credit LossesNet Amortized CostGross
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 securities179.2 — 179.2 0.2 (24.2)155.2 
Obligations of U.S. government agencies354.5 — 354.5 — (42.4)312.1 
U.S. agency commercial mortgage-backed securities510.5 — 510.5 — (52.9)457.6 
U.S. agency residential mortgage-backed securities1,232.6 — 1,232.6 — (137.0)1,095.6 
Collateralized mortgage obligations475.9 — 475.9 0.2 (69.0)407.1 
Corporate securities57.0 (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 $18.1 million of unamortized losses related to the 2021 and 2022 transfer of securities from available-for-sale to held-to-maturity.
Gross Unrealized Losses and Fair Values of Investment Securities
The following tables show the gross unrealized losses and fair values of available-for-sale investment securities and the length of time individual investment securities have been in an unrealized loss position as of September 30, 2024 and December 31, 2023.
 Less than 12 Months12 Months or MoreTotal
September 30, 2024Fair
Value
Gross
Unrealized
Losses
Fair
Value
Gross
Unrealized
Losses
Fair
Value
Gross
Unrealized
Losses
Available-for-Sale:      
U.S. Treasury notes$— $— $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 securities0.1 — 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 obligations3.2 — 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 $(338.3)
 Less than 12 Months12 Months or MoreTotal
December 31, 2023Fair
Value
Gross
Unrealized
Losses
Fair
Value
Gross
Unrealized
Losses
Fair
Value
Gross
Unrealized
Losses
Available-for-Sale:      
U.S. Treasury notes$— $— $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 securities0.2 — 1,083.1 (87.7)1,083.3 (87.7)
U.S. agency residential mortgage-backed securities0.7 — 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, 2024AAAAAABBBBBNot RatedTotal
U.S. Treasury notes$99.6 $— $— $— $— $— $99.6 
State, county, and municipal securities85.7 74.2 10.1 — — 6.7 176.7 
Obligations of U.S. government agencies356.4 — — — — — 356.4 
U.S. agency commercial mortgage-backed securities
FNMA/FHLMC349.7 — — — — — 349.7 
GNMA143.3 — — — — — 143.3 
U.S. agency residential mortgage-backed securities
FNMA/FHLMC1,080.7 — — — — — 1,080.7 
GNMA38.8 — — — — — 38.8 
Collateralized mortgage obligations
FNMA/FHLMC308.5 — — — — — 308.5 
GNMA131.3 — — — — — 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, 2023AAAAAABBBBBNot RatedTotal
U.S. Treasury notes$399.0 $— $— $— $— $— $399.0 
State, county, and municipal securities69.0 92.3 10.6 — — 7.3 179.2 
Obligations of U.S. government agencies354.5 — — — — — 354.5 
U.S. agency commercial mortgage-backed securities
FNMA/FHLMC359.7 — — — — — 359.7 
GNMA150.8 — — — — — 150.8 
U.S. agency residential mortgage-backed securities
FNMA/FHLMC1,189.8 — — — — — 1,189.8 
GNMA42.8 — — — — — 42.8 
Collateralized mortgage obligations
FNMA/FHLMC334.1 — — — — — 334.1 
GNMA141.8 — — — — — 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-SaleHeld-to-Maturity
September 30, 2024Amortized
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 years1,326.9 1,265.9 416.0 396.8 
After five years but within ten years1,166.4 1,095.1 485.5 437.5 
After ten years3,284.0 3,085.8 1,739.4 1,545.6 
Total$5,866.0 $5,534.3 $2,742.0 $2,480.5