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Securities (Tables)
 3 Months Ended
Mar. 31, 2021
Investments, Debt and Equity Securities [Abstract]  
Summary of Amortized Cost and Fair Value of Available-for-sale and Held-to-maturity Securities
The amortized cost and fair value of securities available-for-sale and held-to-maturity at March 31, 2021 and December 31, 2020 are summarized as follows (in thousands):
 Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Fair
Value
March 31, 2021:    
Securities available-for-sale:    
U.S. Treasury securities$83,971 $22 $— $83,993 
U.S. government agency securities159,613 1,224 3,381 157,456 
Mortgage-backed securities1,583,670 40,921 14,813 1,609,778 
State and municipal securities1,480,691 40,140 8,471 1,512,360 
Asset-backed securities191,005 382 692 190,695 
Corporate notes and other121,766 2,824 1,853 122,737 
 $3,620,716 $85,513 $29,210 $3,677,019 
Securities held-to-maturity:    
State and municipal securities$1,014,543 $22,320 $3,114 $1,033,749 
 $1,014,543 $22,320 $3,114 $1,033,749 
Allowance for credit losses - securities held-to-maturity(198)
Securities held-to-maturity, net of allowance for credit losses$1,014,345 
December 31, 2020:    
Securities available-for-sale:    
U.S. Treasury securities$82,199 $10 $— $82,209 
U.S. government agency securities74,916 1,547 60 76,403 
Mortgage-backed securities1,623,759 67,759 2,327 1,689,191 
State and municipal securities1,411,288 44,559 12,484 1,443,363 
Asset-backed securities177,878 715 657 177,936 
Corporate notes and other117,256 2,632 2,309 117,579 
 $3,487,296 $117,222 17,837 $3,586,681 
Securities held-to-maturity:    
State and municipal securities$1,028,550 $38,272 $291 $1,066,531 
$1,028,550 $38,272 $291 $1,066,531 
Allowance for credit losses - securities held-to-maturity(191)
Securities held-to-maturity, net of allowance for credit losses$1,028,359 
Amortized Cost and Fair Value of Debt Securities by Contractual Maturity
The amortized cost and fair value of debt securities as of March 31, 2021 by contractual maturity are 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, 2021:Amortized
Cost		Fair
Value		Amortized
 Cost		Fair
Value
Due in one year or less$88,478 $88,502 $— $— 
Due in one year to five years13,022 13,266 1,409 1,476 
Due in five years to ten years265,259 278,713 7,152 7,223 
Due after ten years1,479,282 1,496,065 1,005,982 1,025,050 
Mortgage-backed securities1,583,670 1,609,778 — — 
Asset-backed securities191,005 190,695 — — 
 $3,620,716 $3,677,019 $1,014,543 $1,033,749 
Classification of Investments According to Term of Unrealized Losses of Less than Twelve Months or Twelve Months or Longer
At March 31, 2021 and December 31, 2020, the following investments 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, 2021      
U.S. Treasury securities$— $— $— $— $— $— 
U.S. government agency securities109,710 3,355 5,701 26 115,411 3,381 
Mortgage-backed securities427,992 14,434 30,072 379 458,064 14,813 
State and municipal securities287,208 4,779 253,998 4,379 541,206 9,158 
Asset-backed securities62,619 662 13,228 30 75,847 692 
Corporate notes13,995 5 25,450 1,848 39,445 1,853 
Total temporarily-impaired securities$901,524 $23,235 $328,449 $6,662 $1,229,973 $29,897 
At December 31, 2020      
U.S. Treasury securities$— $— $— $— $— $— 
U.S. government agency securities9,962 38 6,091 22 16,053 60 
Mortgage-backed securities165,696 1,772 35,997 555 201,693 2,327 
State and municipal securities175,115 2,220 345,435 10,264 520,550 12,484 
Asset-backed securities46,399 207 52,840 450 99,239 657 
Corporate notes9,978 40 23,920 2,269 33,898 2,309 
Total temporarily-impaired securities$407,150 $4,277 $464,283 $13,560 $871,433 $17,837