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Investment Securities (Tables)
 12 Months Ended
Dec. 31, 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:
December 31, 2024Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair
Value
Available-for-Sale:
U.S. Treasury notes$245.0 $— $(18.1)$226.9 
State, county, and municipal securities252.0 — (40.0)212.0 
Obligations of U.S. government agencies224.1 — (10.8)213.3 
U.S. agency commercial mortgage-backed securities1,057.2 0.1 (74.5)982.8 
U.S. agency residential mortgage-backed securities1,301.2 0.4 (125.8)1,175.8 
U.S. agency collateralized mortgage obligations1,156.9 0.6 (106.6)1,050.9 
Private mortgage-backed securities218.1 — (27.6)190.5 
Collateralized loan obligation770.7 1.3 — 772.0 
Corporate securities249.6 — (16.7)232.9 
Total$5,474.8 $2.4 $(420.1)$5,057.1 

December 31, 2024
Amortized
Cost(1)
Gross Unrealized GainsGross
Unrealized
Losses		Estimated
Fair
Value
Held-to Maturity:
U.S. Treasury notes$99.8 $— $(0.4)$99.4 
State, county, and municipal securities176.3 0.2 (26.7)149.8 
Obligations of U.S. government agencies470.1 — (59.9)410.2 
U.S. agency commercial mortgage-backed securities374.9 — (33.4)341.5 
U.S. agency residential mortgage-backed securities1,082.5 — (140.7)941.8 
U.S. agency collateralized mortgage obligations427.8 — (66.1)361.7 
Corporate securities57.0 — (2.8)54.2 
Total$2,688.4 $0.2 $(330.0)$2,358.6 
(1) Amortized cost presented above is net of an ACL of $0.9 million and includes $8.0 million of unamortized gains and $15.9 million of unamortized losses related to the 2021 and 2022 transfer of securities from available-for-sale to held-to-maturity, respectively.
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 
U.S. agency 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
Cost(1)
Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair
Value
Held-to Maturity:
U.S. Treasury notes$399.0 $— $(2.8)$396.2 
State, county, and municipal securities179.2 0.2 (24.2)155.2 
Obligations of U.S. government agencies354.5 — (42.4)312.1 
U.S. agency commercial mortgage-backed securities510.5 — (52.9)457.6 
U.S. agency residential mortgage-backed securities1,232.6 — (137.0)1,095.6 
U.S. agency collateralized mortgage obligations475.9 0.2 (69.0)407.1 
Corporate securities57.0 — (6.8)50.2 
Total$3,208.7 $0.4 $(335.1)$2,874.0 
(1) Amortized cost presented above is net of an ACL of $0.8 million and includes $10.7 million of unamortized gains and $18.1 million of unamortized losses related to the 2021 and 2022 transfer of securities from available-for-sale to held-to-maturity, respectively.
Gross Unrealized Losses and Fair Values of Investment Securities
The following tables show the gross unrealized losses and fair values of available-for-sale investment securities and the length of time individual investment securities have been in an unrealized loss position as of December 31, 2024 and 2023.
 Less than 12 Months12 Months or MoreTotal
December 31, 2024Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses
Available-for-Sale:      
U.S. Treasury notes$— $— $226.8 $(18.1)$226.8 $(18.1)
State, county, and municipal securities— — 210.1 (40.0)210.1 (40.0)
Obligations of U.S. government agencies3.6 — 206.2 (10.8)209.8 (10.8)
U.S. agency commercial mortgage-backed securities38.1 (0.4)934.9 (74.1)973.0 (74.5)
U.S. agency residential mortgage-backed securities33.7 (1.5)1,086.2 (124.3)1,119.9 (125.8)
U.S. agency collateralized mortgage obligations20.5 (0.3)987.1 (106.3)1,007.6 (106.6)
Private mortgage-backed securities— — 190.4 (27.6)190.4 (27.6)
Collateralized loan obligation10.0 — — — 10.0 — 
Corporate securities27.5 (0.5)205.5 (16.2)233.0 (16.7)
Total$133.4 $(2.7)$4,047.2 $(417.4)$4,180.6 $(420.1)
 Less than 12 Months12 Months or MoreTotal
December 31, 2023Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses
Available-for-Sale:      
U.S. Treasury notes$— $— $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)
U.S. agency 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, Allowance for Credit Loss
Maturities of Investment Securities
 Available-for-SaleHeld-to-Maturity
December 31, 2024Amortized
Cost		Estimated
Fair Value		Amortized
Cost		Estimated
Fair Value
Within one year$64.7 $63.9 $101.2 $100.7 
After one year but within five years1,338.6 1,258.9 435.9 408.5 
After five years but within ten years1,131.4 1,048.9 471.3 413.6 
After ten years2,940.1 2,685.4 1,680.0 1,435.8 
Total$5,474.8 $5,057.1 $2,688.4 $2,358.6