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Investment Securities (Tables)
 12 Months Ended
Dec. 31, 2023
Investments, Debt and Equity Securities [Abstract]  
Schedule of Amortized Cost and Estimated Fair Value of Securities
The amortized cost and estimated fair value of AFS investment securities were as follows:
(Dollars in thousands)Amortized
Cost		Gross Unrealized
Gain		Gross Unrealized
Loss		Estimated
Fair Value
AFS investment securities:
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 
December 31, 2022
U.S. Treasury$49,156 $— $(2,139)$47,017 
Agency485,331 — (53,893)431,438 
Corporate586,652 — (44,104)542,548 
Collateralized mortgage obligations829,928 — (65,699)764,229 
Mortgage-backed securities953,678 — (137,897)815,781 
Total AFS investment securities$2,904,745 $— $(303,732)$2,601,013 

The carrying amount and estimated fair value of 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:    
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 
December 31, 2022
Municipal bonds$1,148,055 $(43)$1,148,012 $44 $(257,430)$890,626 
Mortgage-backed securities231,692 — 231,692 — (33,621)198,071 
Other8,399 — 8,399 — — 8,399 
Total HTM investment securities$1,388,146 $(43)$1,388,103 $44 $(291,051)$1,097,096 
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.

 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)
 December 31, 2022
 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. Treasury5 $33,982 $(237)1 $13,036 $(1,902)6 $47,018 $(2,139)
Agency8 79,895 (1,265)35 351,543 (52,628)43 431,438 (53,893)
Corporate35 327,984 (20,840)21 214,565 (23,264)56 542,549 (44,104)
Collateralized mortgage obligations41 522,955 (33,318)40 241,229 (32,381)81 764,184 (65,699)
Mortgage-backed securities13 91,762 (6,987)68 724,019 (130,910)81 815,781 (137,897)
Total AFS investment securities102 $1,056,578 $(62,647)165 $1,544,392 $(241,085)267 $2,600,970 $(303,732)
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:
For the Year Ended December 31, 2023
(Dollars in thousands)Balance, December 31, 2022Provision for Credit Losses Balance, December 31, 2023
HTM Investment securities:
Municipal bonds$43 $83 $126 
For the Year Ended December 31, 2022
(Dollars in thousands)Balance at December 31, 2021Provision for Credit Losses Balance, December 31, 2022
HTM Investment securities:
Municipal bonds$22 $21 $43 
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 (losses) gains for the periods indicated:

For the Year Ended December 31,
(Dollars in thousands)202320222021
Amortized cost of AFS investment securities sold$1,822,531 $934,703 $884,231 
Gross realized gains$986 $13,645 $23,185 
Gross realized losses(254,913)(11,935)(6,279)
Net realized gains on sales of AFS investment securities$(253,927)$1,710 $16,906 
Schedule of Amortized Cost and Estimated Fair Value of Investment Securities Available for Sale by Contractual Maturity
The amortized cost and fair value of investment securities at December 31, 2023, 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:          
Treasury$538,899 $539,256 $— $— $— $— $— $— $538,899 $539,256 
Agency— — — — 1,190 1,149 751 719 1,941 1,868 
Corporate13,000 12,951 254,872 251,485 213,627 181,907 — — 481,499 446,343 
Collateralized mortgage obligations— — 41,266 41,112 63,988 63,288 48,447 48,204 153,701 152,604 
Total AFS investment securities551,899 552,207 296,138 292,597 278,805 246,344 49,198 48,923 1,176,040 1,140,071 
HTM investment securities:          
Municipal bonds— — 26,257 24,543 37,985 33,749 1,082,002 882,284 1,146,244 940,576 
Collateralized mortgage obligations— — 101 101 — — 334,896 326,891 334,997 326,992 
Mortgage-backed securities— — 1,913 2,001 3,482 3,593 226,762 196,075 232,157 201,669 
Other— — — — — — 16,269 16,269 16,269 16,269 
Total HTM investment securities— — 28,271 26,645 41,467 37,342 1,659,929 1,421,519 1,729,667 1,485,506 
Total investment securities$551,899 $552,207 $324,409 $319,242 $320,272 $283,686 $1,709,127 $1,470,442 $2,905,707 $2,625,577