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Investment Securities (Tables)
 3 Months Ended
Mar. 31, 2024
Investments, Debt and Equity Securities [Abstract]  
Schedule of Amortized Cost and Estimated Fair Value of Securities
The amortized cost and estimated fair value of available-for-sale (“AFS”) investment securities were as follows:
(Dollars in thousands)Amortized
 Cost		Gross Unrealized
Gain		Gross Unrealized
Loss		Estimated
Fair Value
AFS investment securities:
March 31, 2024    
U.S. Treasury$565,128 $4 $(193)$564,939 
Agency1,734 — (64)1,670 
Corporate471,426 243 (32,787)438,882 
Collateralized mortgage obligations149,765 — (1,235)148,530 
Total AFS investment securities$1,188,053 $247 $(34,279)$1,154,021 
December 31, 2023
U.S. Treasury$538,899 $381 $(24)$539,256 
Agency1,941 — (73)1,868 
Corporate481,499 52 (35,208)446,343 
Collateralized mortgage obligations153,701 — (1,097)152,604 
Total AFS investment securities$1,176,040 $433 $(36,402)$1,140,071 

The carrying amount and estimated fair value of held-to-maturity (“HTM”) investment securities were as follows:
(Dollars in thousands)Amortized
 Cost		Allowance for Credit LossesNet Carrying AmountGross Unrecognized
Gain		Gross Unrecognized
Loss		Estimated
Fair Value
HTM investment securities:
March 31, 2024
Municipal bonds$1,145,902 $(115)$1,145,787 $264 $(228,193)$917,858 
Collateralized mortgage obligations329,837 — 329,837 1,288 (10,867)320,258 
Mortgage-backed securities228,611 — 228,611 146 (35,422)193,335 
Other16,246 — 16,246 — — 16,246 
Total HTM investment securities$1,720,596 $(115)$1,720,481 $1,698 $(274,482)$1,447,697 
December 31, 2023
Municipal bonds$1,146,244 $(126)$1,146,118 $819 $(206,361)$940,576 
Collateralized mortgage obligations334,997 — 334,997 1,565 (9,570)326,992 
Mortgage-backed securities232,157 — 232,157 310 (30,798)201,669 
Other16,269 — 16,269 — — 16,269 
Total HTM investment securities$1,729,667 $(126)$1,729,541 $2,694 $(246,729)$1,485,506 
Schedule of Number, Fair Value and Gross Unrealized Holding Losses of the Company's Investment Securities by Investment Category and Length of Time that the Securities have been in a Continuous Loss Position
The table below summarizes the number, fair value, and gross unrealized holding losses of the Company’s AFS investment securities in an unrealized loss position for which an allowance for credit losses has not been recorded as of the dates indicated, aggregated by investment category and length of time in a continuous loss position.
 March 31, 2024
 Less than 12 Months12 Months or LongerTotal
(Dollars in thousands)NumberFair
Value		Gross
Unrealized
Losses		NumberFair
Value		Gross
Unrealized
Losses		NumberFair
Value		Gross
Unrealized
Losses
AFS investment securities:
U.S. Treasury21 $515,076 $(193)— $— $— 21 $515,076 $(193)
Agency— — — 4 1,670 (64)4 1,670 (64)
Corporate— — — 41 365,603 (32,787)41 365,603 (32,787)
Collateralized mortgage obligations— — — 28 148,530 (1,235)28 148,530 (1,235)
Total AFS investment securities21 $515,076 $(193)73 $515,803 $(34,086)94 $1,030,879 $(34,279)
 December 31, 2023
 Less than 12 Months12 Months or LongerTotal
(Dollars in thousands)NumberFair
Value		Gross
Unrealized
Losses		NumberFair
Value		Gross
Unrealized
Losses		NumberFair
Value		Gross
Unrealized
Losses
AFS investment securities:
U.S. Treasury4 $98,622 $(24)— $— $— 4 $98,622 $(24)
Agency— — — 4 1,868 (73)4 1,868 (73)
Corporate1 4,989 (3)47 431,353 (35,205)48 436,342 (35,208)
Collateralized mortgage obligations— — — 28 152,604 (1,097)28 152,604 (1,097)
Total AFS investment securities5 $103,611 $(27)79 $585,825 $(36,375)84 $689,436 $(36,402)
Schedule of Allowance for Credit Losses on Company's Held-to-maturity Debt Securities The following table presents a rollforward by major security type of the ACL on the Company's HTM debt securities as of and for the periods indicated:
Three Months Ended March 31, 2024
(Dollars in thousands)
 Balance,
December 31, 2023
Provision (Recapture) for Credit Losses
Balance,
March 31, 2024
HTM investment securities:
Municipal bonds$126 $(11)$115 
Three Months Ended March 31, 2023
(Dollars in thousands)
 Balance,
December 31, 2022
Provision for Credit Losses
Balance,
March 31, 2023
HTM investment securities:
Municipal bonds$43 $184 $227 
Schedule of Realized Gain (Loss) on Investments
The following table presents the amortized cost of securities sold with related gross realized gains, gross realized losses, and net realized gains for the periods indicated:
Three Months Ended
March 31,March 31,
(Dollars in thousands)20242023
Amortized cost of AFS investment securities sold$— $304,182 
Gross realized gains$— $986 
Gross realized (losses)— (848)
Net realized gains on sales of AFS investment securities
$— $138 
Schedule of Amortized Cost and Estimated Fair Value of Investment Securities Available for Sale by Contractual Maturity
The amortized cost and estimated fair value of investment securities at March 31, 2024, by contractual maturity, are shown in the table below.
Due in One Year
or Less		Due after One Year
through Five Years		Due after Five Years
through Ten Years		Due after
Ten Years		Total
(Dollars in thousands)Amortized
Cost		Fair
Value		Amortized
Cost		Fair
Value		Amortized
Cost		Fair
Value		Amortized
Cost		Fair
Value		Amortized
Cost		Fair
Value
AFS investment securities:          
U.S. Treasury$565,128 $564,939 $— $— $— $— $— $— $565,128 $564,939 
Agency— — 257 250 786 758 691 662 1,734 1,670 
Corporate33,006 32,995 233,484 231,709 204,936 174,178 — — 471,426 438,882 
Collateralized mortgage obligations49 49 40,793 40,660 60,925 60,378 47,998 47,443 149,765 148,530 
Total AFS investment securities598,183 597,983 274,534 272,619 266,647 235,314 48,689 48,105 1,188,053 1,154,021 
HTM investment securities:
Municipal bonds— — 26,337 24,621 38,068 34,154 1,081,497 859,083 1,145,902 917,858 
Collateralized mortgage obligations— — 86 86 — — 329,751 320,172 329,837 320,258 
Mortgage-backed securities— — 1,918 1,977 3,476 3,527 223,217 187,831 228,611 193,335 
Other— — — — — — 16,246 16,246 16,246 16,246 
Total HTM investment securities— — 28,341 26,684 41,544 37,681 1,650,711 1,383,332 1,720,596 1,447,697 
Total investment securities$598,183 $597,983 $302,875 $299,303 $308,191 $272,995 $1,699,400 $1,431,437 $2,908,649 $2,601,718