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Investment Securities (Tables)
 12 Months Ended
Dec. 31, 2022
Investments, Debt and Equity Securities [Abstract]  
Amortized Cost and Approximate Fair Values of Investment Securities
The amortized cost and approximate fair values of investment securities are summarized as follows:
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 Securities272.3 — (30.8)241.5 
Total$7,573.5 $2.3 $(629.7)$6,946.1 
December 31, 2022Amortized
Cost		Allowance for Credit LossesNet Carrying AmountGross Unrealized GainsGross
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 $20.2 million of unamortized losses and $14.2 million of unamortized gains related to the 2021 and 2022 transfer of securities from available-for-sale to held-to-maturity.
December 31, 2021Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair
Value
Available-for-Sale
U.S. Treasury notes$697.6 $— $(12.9)$684.7 
State, county, and municipal securities434.7 2.1 (9.3)427.5 
Obligations of U.S. government agencies356.0 0.1 (9.2)346.9 
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations2,027.3 14.1 (23.3)2,018.1 
Private mortgage-backed securities174.4 0.1 (1.1)173.4 
Collateralized loan obligation898.2 1.2 — 899.4 
Corporate Securities271.1 3.0 (3.6)270.5 
Total$4,859.3 $20.6 $(59.4)$4,820.5 
December 31, 2021Amortized
Cost		Allowance for Credit LossesNet Carrying AmountGross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair
Value
Held-to Maturity
State, county, and municipal securities$67.6 $— $67.6 $2.0 $(0.4)$69.2 
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations1,609.0 — 1,609.0 13.2 (35.3)1,586.9 
Corporate securities11.0 — 11.0 0.4 — 11.4 
Total$1,687.6 $— $1,687.6 $15.6 $(35.7)$1,667.5 
(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 investment securities, aggregated by investment category, and the length of time individual investment securities have been in a continuous unrealized loss position, as of December 31, 2022 and 2021.
 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)
 Less than 12 Months12 Months or MoreTotal
December 31, 2022Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses
Held-to-Maturity      
U.S. Treasury notes$386.4 $(10.2)$— $— $386.4 $(10.2)
State, county and municipal securities33.8 (4.0)96.7 (27.6)130.5 (31.6)
Obligations of U.S. government agencies86.1 (9.9)216.0 (39.7)302.1 (49.6)
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations1,349.4 (156.1)762.5 (145.5)2,111.9 (301.6)
Corporate securities68.2 (7.0)— — 68.2 (7.0)
Total$1,923.9 $(187.2)$1,075.2 $(212.8)$2,999.1 $(400.0)
 Less than 12 Months12 Months or MoreTotal
December 31, 2021Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses
Available-for-Sale      
U.S. Treasury notes$684.7 $(12.9)$— $— $684.7 $(12.9)
State, county, and municipal securities278.7 (9.1)5.0 (0.2)283.7 (9.3)
Obligations of U.S. government agencies297.0 (8.9)16.4 (0.3)313.4 (9.2)
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations1,262.8 (23.0)26.4 (0.3)1,289.2 (23.3)
Private mortgage-backed securities127.2 (1.1)— — 127.2 (1.1)
Corporate securities109.9 (3.3)20.9 (0.3)130.8 (3.6)
Total$2,760.3 $(58.3)$68.7 $(1.1)$2,829.0 $(59.4)
 Less than 12 Months12 Months or MoreTotal
December 31, 2021Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses
Held-to-Maturity      
State, county and municipal securities$29.0 $(0.4)$— $— $29.0 $(0.4)
U.S. agency residential mortgage-backed
   securities & collateralized mortgage
   obligations		1,038.7 (35.3)— — 1,038.7 (35.3)
Total$1,067.7 $(35.7)$— $— $1,067.7 $(35.7)
Debt Securities, Held-to-maturity, Allowance for Credit Loss
The following table presents the activity in the allowance for credit losses related to held-to-maturity securities classified as corporate and state, county, and municipal securities:
Year Ended December 31,202220212020
Beginning balance$— $— $— 
Provision for credit loss expense1.9 — — 
Ending balance of allowance for credit losses$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, 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 
December 31, 2021AAAAAABBBNot RatedTotal
State, county, and municipal securities$17.2 $31.6 $14.7 $— $4.1 $67.6 
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations
FNMA/FHLMC1,439.1 — — — — 1,439.1 
GNMA169.9 — — — — 169.9 
Corporate securities— — 4.0 7.0 — 11.0 
Total$1,626.2 $31.6 $18.7 $7.0 $4.1 $1,687.6 
Maturities of Investment Securities
 Available-for-SaleHeld-to-Maturity
December 31, 2022Amortized
Cost		Estimated
Fair Value		Amortized
Cost		Estimated
Fair Value
Within one year$48.8 $48.1 $2.5 $2.5 
After one year but within five years1,478.9 1,419.8 607.6 581.6 
After five years but within ten years1,750.0 1,564.3 732.7 639.5 
After ten years4,295.8 3,913.9 2,110.9 1,828.6 
Total$7,573.5 $6,946.1 $3,453.7 $3,052.2