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Investment Securities (Tables)
 3 Months Ended
Mar. 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 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, 2023    
U.S. Treasury$14,941 $— $(1,586)$13,355 
Agency464,491 — (46,760)417,731 
Corporate561,380 — (53,191)508,189 
Collateralized mortgage obligations401,105 — (28,473)372,632 
Mortgage-backed securities923,956 — (123,011)800,945 
Total AFS investment securities$2,365,873 $— $(253,021)$2,112,852 
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:
March 31, 2023
Municipal bonds$1,147,822 $(227)$1,147,595 $398 $(223,030)$924,963 
Collateralized mortgage obligations356,435 — 356,435 1,198 (481)357,152 
Mortgage-backed securities228,624 — 228,624 — (30,621)198,003 
Other16,376 — 16,376 — — 16,376 
Total HTM investment securities$1,749,257 $(227)$1,749,030 $1,596 $(254,132)$1,496,494 
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.
 March 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. Treasury14 $13,355 $(1,586)— $— $— 14 $13,355 $(1,586)
Agency13 142,489 (12,151)30 275,242 (34,609)43 417,731 (46,760)
Corporate19 158,469 (14,917)37 349,720 (38,274)56 508,189 (53,191)
Collateralized mortgage obligations20 185,232 (9,004)34 187,400 (19,469)54 372,632 (28,473)
Mortgage-backed securities.17 141,007 (12,813)64 659,938 (110,198)81 800,945 (123,011)
Total AFS investment securities83 $640,552 $(50,471)165 $1,472,300 $(202,550)248 $2,112,852 $(253,021)

 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 allowance for credit losses on the Company's HTM debt securities as of and for the periods indicated:
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 
Three Months Ended March 31, 2022
(Dollars in thousands) Balance,
December 31, 2021		Provision for Credit Losses
Balance,
March 31, 2022
HTM investment securities:
Municipal bonds$22 $19 $41 
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 or losses for the periods indicated:
Three Months Ended
March 31,March 31,
(Dollars in thousands)20232022
Amortized cost of AFS investment securities sold$304,182 $658,505 
Gross realized gains$986 $13,637 
Gross realized (losses)(848)(11,503)
Net realized gains on sales of AFS investment securities$138 $2,134 
Schedule of Amortized Cost and Estimated Fair Value of Investment Securities Available for Sale by Contractual Maturity
The amortized cost and estimated fair value of investment securities at March 31, 2023, by contractual maturity, are shown in the table below.
Due in One Year
or Less		Due after One Year
through Five Years		Due after Five Years
through Ten Years		Due after
Ten Years		Total
(Dollars in thousands)Amortized
Cost		Fair
Value		Amortized
Cost		Fair
Value		Amortized
Cost		Fair
Value		Amortized
Cost		Fair
Value		Amortized
Cost		Fair
Value
AFS investment securities:          
U.S. Treasury$— $— $— $— $14,941 $13,355 $— $— $14,941 $13,355 
Agency17,140 16,966 314,788 292,224 107,278 89,306 25,285 19,235 464,491 417,731 
Corporate— — 288,259 280,121 273,121 228,068 — — 561,380 508,189 
Collateralized mortgage obligations31,382 31,174 71,997 70,731 199,472 178,056 98,254 92,671 401,105 372,632 
Mortgage-backed securities19,902 19,617 14,886 14,656 512,674 452,677 376,494 313,995 923,956 800,945 
Total AFS investment securities68,424 67,757 689,930 657,732 1,107,486 961,462 500,033 425,901 2,365,873 2,112,852 
HTM investment securities:
Municipal bonds— — 10,589 10,224 53,233 47,646 1,084,000 867,093 1,147,822 924,963 
Collateralized mortgage obligations— — 151 151 — — 356,284 357,001 356,435 357,152 
Mortgage-backed securities— — — — — — 228,624 198,003 228,624 198,003 
Other— — — — — — 16,376 16,376 16,376 16,376 
Total HTM investment securities— — 10,740 10,375 53,233 47,646 1,685,284 1,438,473 1,749,257 1,496,494 
Total investment securities$68,424 $67,757 $700,670 $668,107 $1,160,719 $1,009,108 $2,185,317 $1,864,374 $4,115,130 $3,609,346