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Investment Securities (Tables)
 12 Months Ended
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, 2023Amortized
Cost		Allowance for Credit LossesNet Amortized CostGross
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 securities256.7 — 256.7 — (36.9)219.8 
Obligations of U.S. government agencies179.4 — 179.4 — (10.9)168.5 
U.S. agency commercial mortgage-backed securities1,192.7 — 1,192.7 0.6 (87.7)1,105.6 
U.S. agency residential mortgage-backed securities1,496.3 — 1,496.3 1.2 (130.6)1,366.9 
Collateralized mortgage obligations1,308.5 — 1,308.5 1.3 (120.3)1,189.5 
Private mortgage-backed securities241.3 — 241.3 — (30.9)210.4 
Collateralized loan obligations1,121.9 — 1,121.9 0.1 (2.3)1,119.7 
Corporate securities260.8 — 260.8 — (24.4)236.4 
Total$6,307.8 $— $6,307.8 $3.2 $(469.5)$5,841.5 

December 31, 2023
Amortized
Cost1
Allowance for Credit LossesNet Amortized CostGross Unrealized GainsGross
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 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, 2022Amortized
Cost		Allowance for Credit LossesNet Amortized CostGross
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 securities314.3 — 314.3 — (50.6)263.7 
Obligations of U.S. government agencies216.2 — 216.2 — (17.3)198.9 
U.S. agency commercial mortgage-backed securities1,468.0 — 1,468.0 0.2 (117.0)1,351.2 
U.S. agency residential mortgage-backed securities1,726.0 — 1,726.0 — (167.7)1,558.3 
Collateralized mortgage obligations1,491.5 — 1,491.5 — (141.3)1,350.2 
Private mortgage-backed securities264.9 — 264.9 — (36.9)228.0 
Collateralized loan obligations1,145.2 — 1,145.2 — (33.6)1,111.6 
Corporate securities272.3 — 272.3 — (30.8)241.5 
Total$7,573.5 $— $7,573.5 $2.3 $(629.7)$6,946.1 
December 31, 2022
Amortized
Cost1
Allowance for Credit LossesNet Amortized CostGross
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 securities181.2 (0.1)181.1 0.2 (31.6)149.7 
Obligations of U.S. government agencies351.7 — 351.7 — (49.6)302.1 
U.S. agency commercial mortgage-backed securities526.8 — 526.8 — (58.3)468.5 
U.S. agency residential mortgage-backed securities1,391.5 — 1,391.5 — (166.6)1,224.9 
Collateralized mortgage obligations525.8 — 525.8 0.2 (76.7)449.3 
Corporate securities80.1 (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 $14.2 million unamortized gains related to the 2021 and 2022 transfer of securities from available-for-sale to held-to-maturity.
Gross Unrealized Losses and Fair Values of Investment Securities
The following tables show the gross unrealized losses and fair values of available-for-sale investment securities and the length of time individual investment securities have been in an unrealized loss position as of December 31, 2023 and 2022.
 Less than 12 Months12 Months or MoreTotal
December 31, 2023Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses
Available-for-Sale      
U.S. Treasury notes$— $— $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 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 Months12 Months or MoreTotal
December 31, 2022Fair
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 securities104.9 (9.0)146.1 (41.6)251.0 (50.6)
Obligations of U.S. government agencies152.2 (10.1)46.1 (7.2)198.3 (17.3)
U.S. agency commercial mortgage-backed securities1,069.8 (59.9)270.8 (57.0)1,340.6 (116.9)
U.S. agency residential mortgage-backed securities1,213.5 (112.3)344.6 (55.4)1,558.1 (167.7)
Collateralized mortgage obligations1,016.4 (90.1)333.0 (51.3)1,349.4 (141.4)
Private mortgage-backed securities133.1 (19.1)94.9 (17.8)228.0 (36.9)
Collateralized loan obligations1,082.6 (33.1)28.9 (0.5)1,111.5 (33.6)
Corporate securities130.6 (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 31,202320222021
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, 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 
December 31, 2022AAAAAABBBBBNot RatedTotal
U.S. Treasury notes$396.6 $— $— $— $— $— $396.6 
State, county, and municipal securities68.3 92.8 11.5 — — 8.6 181.2 
Obligations of U.S. government agencies351.7 — — — — — 351.7 
U.S. agency commercial mortgage-backed securities
FNMA/FHLMC364.9 — — — — — 364.9 
GNMA161.9 — — — — — 161.9 
U.S. agency residential mortgage-backed securities
FNMA/FHLMC1,342.9 — — — — — 1,342.9 
GNMA48.6 — — — — — 48.6 
Collateralized mortgage obligations
FNMA/FHLMC525.8 — — — — — 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-SaleHeld-to-Maturity
December 31, 2023Amortized
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 years1,004.4 938.7 346.6 329.4 
After five years but within ten years1,373.9 1,255.4 654.0 579.1 
After ten years3,889.4 3,608.0 1,899.0 1,658.0 
Total$6,307.8 $5,841.5 $3,208.7 $2,874.0