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Investment Securities (Tables)
 3 Months Ended
Mar. 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:
March 31, 2023Amortized
Cost		Allowance for Credit LossesNet Amortized CostGross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair
Value
Available-for-Sale:
U.S. Treasury notes$250.1 $— $250.1 $— $(26.9)$223.2 
State, county, and municipal securities262.0 — 262.0 — (40.9)221.1 
Obligations of U.S. government agencies186.4 — 186.4 — (12.3)174.1 
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations4,221.0 — 4,221.0 0.2 (362.8)3,858.4 
Private mortgage-backed securities260.3 — 260.3 — (35.2)225.1 
Collateralized loan obligations1,143.3 1,143.3 — (29.4)1,113.9 
Corporate securities261.7 (2.6)259.1 — (25.6)233.5 
Total$6,584.8 $(2.6)$6,582.2 $0.2 $(533.1)$6,049.3 

March 31, 2023Amortized
Cost		Allowance for Credit LossesNet Amortized CostGross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair
Value
Held-to-Maturity:
U.S. Treasury notes$397.2 $— $397.2 $— $(7.6)$389.6 
State, county, and municipal securities180.9 — 180.9 0.4 (25.2)156.1 
Obligations of U.S. government agencies352.3 — 352.3 — (44.7)307.6 
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations (1)
2,389.4 — 2,389.4 1.0 (269.7)2,120.7 
Corporate securities57.1 (0.7)56.4 — (4.6)51.8 
Total$3,376.9 $(0.7)$3,376.2 $1.4 $(351.8)$3,025.8 
(1) Amortized cost presented above include $19.7 million of unamortized losses and $13.2 million of unamortized gains in U.S. agency residential and commercial mortgage-backed securities and collateralized mortgage obligations related to the 2021 and 2022 transfer of securities from available-for-sale to held-to-maturity.
December 31, 2022Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair
Value
Available-for-Sale:
U.S. Treasury notes$675.1 $2.1 $(34.5)$642.7 
State, county, and municipal securities314.3 — (50.6)263.7 
Obligations of U.S. government agencies216.2 — (17.3)198.9 
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations4,685.5 0.2 (426.0)4,259.7 
Private mortgage-backed securities264.9 — (36.9)228.0 
Collateralized loan obligations1,145.2 — (33.6)1,111.6 
Corporate securities
272.3 — (30.8)241.5 
Total$7,573.5 $2.3 $(629.7)$6,946.1 

December 31, 2022Amortized
Cost		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 residential & commercial mortgage-backed securities & collateralized mortgage obligations(1)
2,444.1 — 2,444.1 0.2 (301.6)2,142.7 
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 include $20.1 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.
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 March 31, 2023 and December 31, 2022.
 Less than 12 Months12 Months or MoreTotal
March 31, 2023Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses
Available-for-Sale:      
U.S. Treasury notes$48.1 $(1.3)$175.1 $(25.6)$223.2 $(26.9)
State, county, and municipal securities8.0 (0.3)210.0 (40.6)218.0 (40.9)
Obligations of U.S. government agencies86.7 (4.0)86.8 (8.3)173.5 (12.3)
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations1,388.6 (63.5)2,459.5 (299.3)3,848.1 (362.8)
Private mortgage-backed securities0.1 — 225.0 (35.2)225.1 (35.2)
Collateralized loan obligations117.3 (3.0)996.6 (26.4)1,113.9 (29.4)
Corporate securities80.0 (2.8)153.5 (22.8)233.5 (25.6)
Total$1,728.8 $(74.9)$4,306.5 $(458.2)$6,035.3 $(533.1)
 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 residential & commercial mortgage-backed securities & collateralized mortgage obligations3,299.7 (262.3)948.4 (163.7)4,248.1 (426.0)
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 investment securities:
Three Months Ended March 31,
20232022
Beginning balance$1.9 $— 
Provision for available-for-sale credit losses2.6 — 
Reversal of held-to-maturity credit loss losses(1.2)1.6 
Ending balance$3.3 $1.6 
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:
March 31, 2023AAAAAABBBBBNot RatedTotal
U.S. Treasury notes$397.2 $— $— $— $— $— $397.2 
State, county, and municipal securities68.2 92.7 11.5 — — 8.5 180.9 
Obligations of U.S. government agencies352.3 — — — — — 352.3 
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations
FNMA/FHLMC2,183.2 — — — — — 2,183.2 
GNMA206.2 — — — — — 206.2 
Corporate securities— — — 52.1 — 5.0 57.1 
Total$3,207.1 $92.7 $11.5 $52.1 $— $13.5 $3,376.9 
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 residential & commercial mortgage-backed securities & collateralized mortgage obligations
FNMA/FHLMC2,233.6 — — — — — 2,233.6 
GNMA210.5 — — — — — 210.5 
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 Maturities of mortgage-backed securities have been adjusted to reflect shorter maturities based upon estimated prepayments of principal. All other investment securities maturities are shown at contractual maturity dates.
 Available-for-SaleHeld-to-Maturity
March 31, 2023Amortized
Cost		Estimated
Fair Value		Amortized
Cost		Estimated
Fair Value
Within one year$0.5 $0.5 $307.2 $300.4 
After one year but within five years786.6 746.1 301.1 287.2 
After five years but within ten years1,686.5 1,524.1 704.1 624.4 
After ten years4,111.2 3,778.6 2,064.5 1,813.8 
Total$6,584.8 $6,049.3 $3,376.9 $3,025.8