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Securities (Tables)
 3 Months Ended
Mar. 31, 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 March 31, 2024 and December 31, 2023 are summarized as follows (in thousands):
 Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Fair
Value
March 31, 2024:    
Securities available-for-sale:    
U.S. Treasury securities$877,149 $21 $4,379 $872,791 
U.S. Government agency securities316,289 20 27,476 288,833 
Mortgage-backed securities1,095,839 544 126,171 970,212 
State and municipal securities1,620,208 26,568 53,993 1,592,783 
Asset-backed securities230,109 452 8,893 221,668 
Corporate notes and other474,229 52 41,850 432,431 
 $4,613,823 $27,657 $262,762 $4,378,718 
 Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Fair
Value
March 31, 2024:    
Securities held-to-maturity:    
U.S. Treasury securities$90,199 $— $3,415 $86,784 
U.S. Government agency securities364,898 — 20,355 344,543 
Mortgage-backed securities378,228 175 39,134 339,269 
State and municipal securities1,878,776 2,783 180,516 1,701,043 
Asset-backed securities197,511 49 12,429 185,131 
Corporate notes and other85,225 — 8,936 76,289 
 $2,994,837 $3,007 $264,785 $2,733,059 
Allowance for credit losses - securities held-to-maturity(1,708)
Securities held-to-maturity, net of allowance for credit losses$2,993,129 
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 March 31, 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
March 31, 2024:Amortized
Cost		Fair
Value		Amortized
 Cost		Fair
Value
Due in one year or less$27,863 $29,143 $95,418 $92,840 
Due in one year to five years241,765 236,003 372,318 347,867 
Due in five years to ten years506,600 462,247 91,872 84,791 
Due after ten years2,511,647 2,459,445 1,859,490 1,683,161 
Mortgage-backed securities1,095,839 970,212 378,228 339,269 
Asset-backed securities230,109 221,668 197,511 185,131 
 $4,613,823 $4,378,718 $2,994,837 $2,733,059 
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair Value
At March 31, 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 March 31, 2024      
U.S. Treasury securities$669,265 $3,780 $130,101 $599 $799,366 $4,379 
U.S. Government agency securities1,428 13 286,532 27,463 287,960 27,476 
Mortgage-backed securities43,474 615 799,688 125,556 843,162 126,171 
State and municipal securities254,269 2,053 500,627 51,940 754,896 53,993 
Asset-backed securities39,732 91 120,109 8,802 159,841 8,893 
Corporate notes26,183 1,099 375,259 40,751 401,442 41,850 
Total temporarily-impaired securities$1,034,351 $7,651 $2,212,316 $255,111 $3,246,667 $262,762 
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