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Investment Securities (Tables)
 9 Months Ended
Sep. 30, 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 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:
September 30, 2023    
U.S. Treasury$14,947 $— $(2,033)$12,914 
Agency457,694 — (52,180)405,514 
Corporate560,873 57 (56,122)504,808 
Collateralized mortgage obligations337,195 — (32,865)304,330 
Mortgage-backed securities840,545 — (153,512)687,033 
Total AFS investment securities$2,211,254 $57 $(296,712)$1,914,599 
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 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:
September 30, 2023
Municipal bonds$1,146,583 $(128)$1,146,455 $— $(321,595)$824,860 
Collateralized mortgage obligations341,238 — 341,238 1,064 (17,902)324,400 
Mortgage-backed securities233,881 — 233,881 — (44,999)188,882 
Other16,292 — 16,292 — — 16,292 
Total HTM investment securities$1,737,994 $(128)$1,737,866 $1,064 $(384,496)$1,354,434 
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.
 September 30, 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. Treasury— $— $— 1 $12,914 $(2,033)1 $12,914 $(2,033)
Agency— — — 40 405,514 (52,180)40 405,514 (52,180)
Corporate1 16,623 (3,377)51 473,191 (52,745)52 489,814 (56,122)
Collateralized mortgage obligations1 4,786 (61)49 299,544 (32,804)50 304,330 (32,865)
Mortgage-backed securities.— — — 79 687,033 (153,512)79 687,033 (153,512)
Total AFS investment securities2 $21,409 $(3,438)220 $1,878,196 $(293,274)222 $1,899,605 $(296,712)
 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:
Three Months Ended September 30, 2023
(Dollars in thousands)
 Balance,
June 30, 2023
Provision for Credit Losses
Balance,
September 30, 2023
HTM investment securities:
Municipal bonds$113 $15 $128 
Three Months Ended September 30, 2022
(Dollars in thousands)
 Balance,
June 30, 2022
Provision for Credit Losses
Balance,
September 30, 2022
HTM investment securities:
Municipal bonds$109 $(18)$91 
Nine Months Ended September 30, 2023
(Dollars in thousands)
 Balance,
December 31, 2022
Provision for Credit Losses
Balance,
September 30, 2023
HTM investment securities:
Municipal bonds$43 $85 $128 
Nine Months Ended September 30, 2022
(Dollars in thousands)
 Balance,
December 31, 2021
Provision for Credit Losses
Balance,
September 30, 2022
HTM investment securities:
Municipal bonds$22 $69 $91 
Schedule of Realized Gain (Loss) on Investments
The following table presents the amortized cost of securities sold with related gross realized gains and losses, as well as net realized gains or losses for the periods indicated:
Three Months EndedNine Months Ended
September 30,September 30,September 30,September 30,
(Dollars in thousands)2023202220232022
Amortized cost of AFS investment securities sold$— $231,076 $304,182 $934,703 
Gross realized gains$— $— $986 $13,645 
Gross realized (losses)— (393)(848)(11,935)
Net realized (losses) gains on sales of AFS investment securities
$— $(393)$138 $1,710 
Schedule of Amortized Cost and Estimated Fair Value of Investment Securities Available for Sale by Contractual Maturity
The amortized cost and estimated fair value of investment securities at September 30, 2023, by contractual maturity, are shown in the table below.
Due in One Year
or Less		Due after One Year
through Five Years		Due after Five Years
through Ten Years		Due after
Ten Years		Total
(Dollars in thousands)Amortized
Cost		Fair
Value		Amortized
Cost		Fair
Value		Amortized
Cost		Fair
Value		Amortized
Cost		Fair
Value		Amortized
Cost		Fair
Value
AFS investment securities:          
U.S. Treasury$— $— $14,947 $12,914 $— $— $— $— $14,947 $12,914 
Agency80,578 78,561 245,493 224,454 106,631 84,760 24,992 17,739 457,694 405,514 
Corporate13,000 12,974 275,068 270,367 272,805 221,467 — — 560,873 504,808 
Collateralized mortgage obligations— — 57,210 56,349 187,343 162,195 92,642 85,786 337,195 304,330 
Mortgage-backed securities5,596 5,538 — — 483,857 410,229 351,092 271,266 840,545 687,033 
Total AFS investment securities99,174 97,073 592,718 564,084 1,050,636 878,651 468,726 374,791 2,211,254 1,914,599 
HTM investment securities:
Municipal bonds— — 26,176 23,682 37,903 32,366 1,082,504 768,812 1,146,583 824,860 
Collateralized mortgage obligations— — 116 116 — — 341,122 324,284 341,238 324,400 
Mortgage-backed securities— — — — 3,489 3,395 230,392 185,487 233,881 188,882 
Other— — — — — — 16,292 16,292 16,292 16,292 
Total HTM investment securities— — 26,292 23,798 41,392 35,761 1,670,310 1,294,875 1,737,994 1,354,434 
Total investment securities$99,174 $97,073 $619,010 $587,882 $1,092,028 $914,412 $2,139,036 $1,669,666 $3,949,248 $3,269,033