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Investment Securities (Tables)
 3 Months Ended
Mar. 31, 2026
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, 2026Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair
Value
Available-for-Sale:
U.S. Treasury notes$246.9 $— $(9.4)$237.5 
State, county, and municipal securities246.7 — (29.6)217.1 
Obligations of U.S. government agencies180.3 — (4.4)175.9 
U.S. agency commercial mortgage-backed securities875.7 0.9 (36.7)839.9 
U.S. agency residential mortgage-backed securities2,301.0 1.9 (79.8)2,223.1 
U.S. agency collateralized mortgage obligations949.0 0.8 (68.9)880.9 
Private mortgage-backed securities188.1 — (19.6)168.5 
Collateralized loan obligation833.8 0.1 (0.5)833.4 
Corporate securities148.5 — (8.2)140.3 
Total$5,970.0 $3.7 $(257.1)$5,716.6 

March 31, 2026
Amortized
Cost(1)
Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair
Value
Held-to-Maturity:
State, county, and municipal securities$173.1 $0.1 $(19.6)$153.6 
Obligations of U.S. government agencies458.6 — (38.5)420.1 
U.S. agency commercial mortgage-backed securities353.3 — (19.3)334.0 
U.S. agency residential mortgage-backed securities908.5 — (85.5)823.0 
U.S. agency collateralized mortgage obligations370.5 0.4 (48.1)322.8 
Corporate securities29.9 — (0.7)29.2 
Total$2,293.9 $0.5 $(211.7)$2,082.7 
(1) Amortized cost presented above is net of an allowance for credit losses of $0.5 million and includes $5.5 million of unamortized gains and $13.1 million of unamortized losses related to the 2021 and 2022 transfer of securities from available-for-sale to held-to-maturity, respectively.
December 31, 2025Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair
Value
Available-for-Sale:
U.S. Treasury notes$246.5 $— $(8.8)$237.7 
State, county, and municipal securities247.3 — (27.2)220.1 
Obligations of U.S. government agencies205.1 — (4.2)200.9 
U.S. agency commercial mortgage-backed securities891.7 1.1 (36.1)856.7 
U.S. agency residential mortgage-backed securities1,834.4 4.4 (68.7)1,770.1 
U.S. agency collateralized mortgage obligations986.1 1.4 (64.8)922.7 
Private mortgage-backed securities193.9 — (19.5)174.4 
Collateralized loan obligation754.3 1.2 — 755.5 
Corporate securities158.7 — (8.7)150.0 
Total$5,518.0 $8.1 $(238.0)$5,288.1 
December 31, 2025
Amortized
Cost(1)
Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair
Value
Held-to-Maturity:
State, county, and municipal securities$173.2 $0.2 $(18.6)$154.8 
Obligations of U.S. government agencies462.3 — (37.7)424.6 
U.S. agency commercial mortgage-backed securities354.8 — (18.8)336.0 
U.S. agency residential mortgage-backed securities941.3 0.1 (84.7)856.7 
U.S. agency collateralized mortgage obligations381.1 0.7 (46.4)335.4 
Corporate securities29.9 — (0.8)29.1 
Total$2,342.6 $1.0 $(207.0)$2,136.6 
(1) Amortized cost presented above is net of an allowance for credit losses of $0.5 million and includes $5.9 million of unamortized gains and $13.7 million of unamortized losses related to the 2021 and 2022 transfer of securities from available-for-sale to held-to-maturity, respectively.
Maturities of Investment Securities
The following schedule represents the amortized cost of debt securities by contractual maturity except for maturities of mortgage-backed securities, which have been adjusted to reflect shorter maturities based upon estimated prepayments of principal.
 Available-for-SaleHeld-to-Maturity
March 31, 2026Amortized
Cost		Estimated
Fair Value		Amortized
Cost		Estimated
Fair Value
Within one year$215.7 $214.0 $46.0 $45.9 
After one year but within five years1,164.4 1,112.7 640.0 605.0 
After five years but within ten years907.2 835.1 400.5 364.1 
After ten years3,682.7 3,554.8 1,207.4 1,067.7 
Total$5,970.0 $5,716.6 $2,293.9 $2,082.7 
Schedule of Unrealized Loss on Investments
The following tables show the gross unrealized losses and fair values of available-for-sale investment securities and the length of time individual investment securities have been in an unrealized loss position as of March 31, 2026 and December 31, 2025.
 Less than 12 Months12 Months or MoreTotal
March 31, 2026Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses
Available-for-Sale:      
U.S. Treasury notes$— $— $237.5 $(9.4)$237.5 $(9.4)
State, county, and municipal securities8.7 — 207.4 (29.6)216.1 (29.6)
Obligations of U.S. government agencies1.7 — 171.2 (4.4)172.9 (4.4)
U.S. agency commercial mortgage-backed securities1.3 — 805.9 (36.7)807.2 (36.7)
U.S. agency residential mortgage-backed securities971.7 (9.9)894.4 (69.9)1,866.1 (79.8)
U.S. agency collateralized mortgage obligations6.2 — 830.4 (68.9)836.6 (68.9)
Private mortgage-backed securities— — 168.4 (19.6)168.4 (19.6)
Collateralized loan obligation459.8 (0.5)— — 459.8 (0.5)
Corporate securities12.5 — 127.8 (8.2)140.3 (8.2)
Total$1,461.9 $(10.4)$3,443.0 $(246.7)$4,904.9 $(257.1)
 Less than 12 Months12 Months or MoreTotal
December 31, 2025Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses
Available-for-Sale:      
U.S. Treasury notes$— $— $237.7 $(8.8)$237.7 $(8.8)
State, county, and municipal securities— — 210.4 (27.2)210.4 (27.2)
Obligations of U.S. government agencies2.4 — 195.8 (4.2)198.2 (4.2)
U.S. agency commercial mortgage-backed securities1.3 — 821.6 (36.1)822.9 (36.1)
U.S. agency residential mortgage-backed securities254.4 (1.1)956.5 (67.6)1,210.9 (68.7)
U.S. agency collateralized mortgage obligations— — 869.9 (64.8)869.9 (64.8)
Private mortgage-backed securities— — 174.3 (19.5)174.3 (19.5)
Corporate securities— — 150.0 (8.7)150.0 (8.7)
Total$258.1 $(1.1)$3,616.2 $(236.9)$3,874.3 $(238.0)