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Securities (Tables)
 9 Months Ended
Sep. 30, 2024
Investments, Debt and Equity Securities [Abstract]  
Debt Securities, Available-for-sale
The amortized cost and fair value of securities available-for-sale and held-to-maturity at September 30, 2024 and December 31, 2023 are summarized as follows (in thousands):
 Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Fair
Value
September 30, 2024:    
Securities available-for-sale:    
U.S. Treasury securities$1,456,514 $17 $24,974 $1,431,557 
U.S. Government agency securities238,930 5 18,132 220,803 
Mortgage-backed securities1,729,398 10,878 85,919 1,654,357 
State and municipal securities1,526,844 11,819 42,231 1,496,432 
Asset-backed securities127,039 1,647 74 128,612 
Corporate notes and other481,651 1,973 24,397 459,227 
 $5,560,376 $26,339 $195,727 $5,390,988 
Securities held-to-maturity:    
U.S. Treasury securities$69,974 $— $1,348 $68,626 
U.S. Government agency securities340,158 — 10,979 329,179 
Mortgage-backed securities370,433 767 25,909 345,291 
State and municipal securities1,863,397 5,005 144,265 1,724,137 
Asset-backed securities176,557 63 6,801 169,819 
Corporate notes and other83,441 — 6,592 76,849 
 $2,903,960 $5,835 $195,894 $2,713,901 
Allowance for credit losses - securities held-to-maturity(1,707)
Securities held-to-maturity, net of allowance for credit losses$2,902,253 
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Fair
Value
December 31, 2023:    
Securities available-for-sale:    
U.S. Treasury securities$907,990 $2 $14,580 $893,412 
U.S. Government agency securities284,607 — 21,877 262,730 
Mortgage-backed securities1,071,963 444 125,017 947,390 
State and municipal securities1,604,874 26,129 45,108 1,585,895 
Asset-backed securities201,577 338 10,280 191,635 
Corporate notes and other477,761 69 41,362 436,468 
 $4,548,772 $26,982 258,224 $4,317,530 
Securities held-to-maturity:    
U.S Treasury securities$90,309 $— $3,840 $86,469 
U.S. Government agency securities364,769 — 19,187 345,582 
Mortgage-backed securities382,100 637 34,900 347,837 
State and municipal securities1,886,459 6,079 159,027 1,733,511 
Asset-backed securities198,418 — 14,228 184,190 
Corporate notes86,009 — 8,414 77,595 
$3,008,064 $6,716 $239,596 $2,775,184 
Allowance for credit losses - securities held-to-maturity(1,707)
Securities held-to-maturity, net of allowance for credit losses$3,006,357 
Amortized Cost and Fair Value of Debt Securities by Contractual Maturity
The amortized cost and fair value of debt securities as of September 30, 2024 by contractual maturity is shown below. Actual maturities may differ from contractual maturities of mortgage- and asset-backed securities since the mortgages and assets underlying the securities may be called or prepaid with or without penalty. Therefore, these securities are not included in the maturity categories in the following summary (in thousands):
 Available-for-saleHeld-to-maturity
September 30, 2024:Amortized
Cost		Fair
Value		Amortized
 Cost		Fair
Value
Due in one year or less$5,075 $9,166 $138,251 $136,597 
Due in one year to five years227,176 214,064 307,863 294,076 
Due in five years to ten years452,701 425,872 80,695 75,397 
Due after ten years3,018,987 2,958,917 1,830,161 1,692,721 
Mortgage-backed securities1,729,398 1,654,357 370,433 345,291 
Asset-backed securities127,039 128,612 176,557 169,819 
 $5,560,376 $5,390,988 $2,903,960 $2,713,901 
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair Value
At September 30, 2024 and December 31, 2023, the following available-for-sale securities had unrealized losses. The table below classifies these investments according to the term of the unrealized losses of less than twelve months or twelve months or longer (in thousands):
 Investments with an Unrealized Loss of
less than 12 months		Investments with an Unrealized Loss of
12 months or longer		Total Investments with an
Unrealized Loss
 Fair ValueUnrealized LossesFair ValueUnrealized LossesFair ValueUnrealized
Losses
At September 30, 2024      
U.S. Treasury securities$626,281 $7,625 $796,446 $17,349 $1,422,727 $24,974 
U.S. Government agency securities— — 219,712 18,132 219,712 18,132 
Mortgage-backed securities391,774 3,313 617,752 82,606 1,009,526 85,919 
State and municipal securities144,268 10,194 534,808 32,037 679,076 42,231 
Asset-backed securities20,457 29 1,455 45 21,912 74 
Corporate notes34,070 1,075 280,418 23,322 314,488 24,397 
Total temporarily-impaired securities$1,216,850 $22,236 $2,450,591 $173,491 $3,667,441 $195,727 
At December 31, 2023      
U.S. Treasury securities$693,621 $11,651 $192,500 $2,929 $886,121 $14,580 
U.S. Government agency securities14,989 11 247,648 21,866 262,637 21,877 
Mortgage-backed securities72,907 1,518 828,251 123,499 901,158 125,017 
State and municipal securities185,108 908 449,212 44,200 634,320 45,108 
Asset-backed securities42,207 254 122,469 10,026 164,676 10,280 
Corporate notes12,679 7 403,882 41,355 416,561 41,362 
Total temporarily-impaired securities$1,021,511 $14,349 $2,243,962 $243,875 $3,265,473 $258,224