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Investment Securities (Tables)
 12 Months Ended
Dec. 31, 2021
Investments, Debt and Equity Securities [Abstract]  
Schedule of amortized cost and estimated fair value of securities
The amortized cost and estimated fair value of investment securities available-for-sale were as follows:
(Dollars in thousands)Amortized
Cost		Gross Unrealized
Gain		Gross Unrealized
Loss		Estimated
Fair Value
Investment securities available-for-sale:
December 31, 2021    
U.S. Treasury$57,708 $614 $(456)$57,866 
Agency440,183 2,081 (10,129)432,135 
Corporate451,621 6,096 (3,856)453,861 
Municipal bonds1,061,985 32,209 (4,281)1,089,913 
Collateralized mortgage obligations680,686 2,012 (6,055)676,643 
Mortgage-backed securities1,586,406 3,220 (26,180)1,563,446 
Total investment securities available-for-sale$4,278,589 $46,232 $(50,957)$4,273,864 
December 31, 2020
U.S. Treasury$30,153 $2,380 $— $32,533 
Agency666,702 24,292 (608)690,386 
Corporate412,223 3,591 (506)415,308 
Municipal bonds1,412,012 37,260 (3,253)1,446,019 
Collateralized mortgage obligations513,259 819 (712)513,366 
Mortgage-backed securities812,384 21,662 (543)833,503 
Total investment securities available-for-sale$3,846,733 $90,004 $(5,622)$3,931,115 

The carrying amount and estimated fair value of investment securities held-to-maturity were as follows:
(Dollars in thousands)Amortized
Cost		Allowance for Credit LossesNet Carrying AmountGross Unrecognized
Gain		Gross Unrecognized
Loss		Estimated
Fair Value
Investment securities held-to-maturity:    
December 31, 2021
Municipal bonds$368,344 $(22)$368,322 $3,834 $(1,649)$370,507 
Mortgage-backed securities11,843 — 11,843 564 — 12,407 
Other1,509 — 1,509 — — 1,509 
Total investment securities held-to-maturity$381,696 $(22)$381,674 $4,398 $(1,649)$384,423 
December 31, 2020
Mortgage-backed securities$22,124 $— $22,124 $1,281 $— $23,405 
Other1,608 — 1,608 — — 1,608 
Total investment securities held-to-maturity$23,732 $— $23,732 $1,281 $— $25,013 
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 shows the number, fair value, and gross unrealized holding losses of the Company’s investment securities available-for-sale 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, 2021
 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
Investment securities available-for-sale:
U.S. Treasury3 $47,235 $(456)— $— $— 3 $47,235 $(456)
Agency19 278,078 (5,634)16 119,750 (4,495)35 397,828 (10,129)
Corporate17 166,563 (849)3 57,274 (3,007)20 223,837 (3,856)
Municipal bonds36 277,564 (4,079)2 6,596 (202)38 284,160 (4,281)
Collateralized mortgage obligations26 226,763 (3,738)15 121,185 (2,317)41 347,948 (6,055)
Mortgage-backed securities103 1,306,455 (20,417)15 173,121 (5,763)118 1,479,576 (26,180)
Total investment securities available-for-sale204 $2,302,658 $(35,173)51 $477,926 $(15,784)255 $2,780,584 $(50,957)
 December 31, 2020
 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
Investment securities available-for-sale:
Agency4 $74,194 $(307)9 $10,434 $(301)13 $84,628 $(608)
Corporate9 71,226 (506)— — — 9 71,226 (506)
Municipal bonds56 312,894 (3,253)— — — 56 312,894 (3,253)
Collateralized mortgage obligations21 215,603 (710)1 431 (2)22 216,034 (712)
Mortgage-backed securities16 139,071 (543)— — — 16 139,071 (543)
Total investment securities available-for-sale106 $812,988 $(5,319)10 $10,865 $(303)116 $823,853 $(5,622)
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 held-to-maturity debt securities as of, and for the periods indicated:

For the Year Ended December 31, 2021
(Dollars in thousands)Balance, December 31, 2020Provision for Credit Losses Balance, December 31, 2021
Investment securities held-to-maturity:
Municipal bonds$— $22 $22 
Schedule of investment securities by external credit rating
The following table summarizes the Company’s investment securities portfolio by Moody’s external rating equivalent and by vintage as of December 31, 2021:
Vintage
(Dollars in thousands)20212020201920182017PriorTotal
Investment securities available-for-sale at fair value:
U.S. Treasury
Aaa - Aa3$32,727 $14,508 $— $10,631 $— $— $57,866 
Agency
Aaa - Aa324,216 296,403 26,017 59,931 — 12,538 419,105 
Baa1 - Baa3— — — 5,410 — 7,620 13,030 
Corporate debt
A1 - A3136,607 67,326 — — 17,938 — 221,871 
Baa1 - Baa346,492 100,709 71,242 — — 13,547 231,990 
Municipal bonds
Aaa - Aa3118,922 563,238 291,483 32,483 50,149 32,780 1,089,055 
A1 - A3— — — — — 858 858 
Collateralized mortgage obligations
Aaa - Aa3268,693 207,155 75,834 16,710 13,274 94,977 676,643 
Mortgage-backed securities
Aaa - Aa31,109,054 358,114 68,606 — 19,777 7,895 1,563,446 
Total investment securities available-for-sale$1,736,711 $1,607,453 $533,182 $125,165 $101,138 $170,215 $4,273,864 
Vintage
(Dollars in thousands)20212020201920182017PriorTotal
Investment securities held-to-maturity at amortized cost:
Municipal bonds
Aaa - Aa3$48,540 $319,804 $— $— $— $— $368,344 
Mortgage-backed securities
Aaa - Aa3— — — 3,425 3,325 5,093 11,843 
Other
Baa1 - Baa3— — — 591 — 918 1,509 
Total investment securities held-to-maturity48,540 319,804 — 4,016 3,325 6,011 381,696 
Total investment securities$1,785,251 $1,927,257 $533,182 $129,181 $104,463 $176,226 $4,655,560 
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 available-for-sale at December 31, 2021, 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
Investment securities available-for-sale:          
Treasury$— $— $10,017 $10,631 $47,691 $47,235 $— $— $57,708 $57,866 
Agency— — 298,491 294,580 97,882 95,525 43,810 42,030 440,183 432,135 
Corporate— — 61,302 61,154 390,319 392,707 — — 451,621 453,861 
Municipal bonds— — 3,129 3,289 90,884 91,329 967,972 995,295 1,061,985 1,089,913 
Collateralized mortgage obligations— — 32,960 32,900 215,306 211,057 432,420 432,686 680,686 676,643 
Mortgage-backed securities— — 2,074 2,156 629,678 624,990 954,654 936,300 1,586,406 1,563,446 
Total investment securities available-for-sale— — 407,973 404,710 1,471,760 1,462,843 2,398,856 2,406,311 4,278,589 4,273,864 
Investment securities held-to-maturity:          
Municipal bonds— — — — — — 368,344 370,507 368,344 370,507 
Mortgage-backed securities— — — — — — 11,843 12,407 11,843 12,407 
Other— — — — — — 1,509 1,509 1,509 1,509 
Total investment securities held-to-maturity— — — — — — 381,696 384,423 381,696 384,423 
Total investment securities$— $— $407,973 $404,710 $1,471,760 $1,462,843 $2,780,552 $2,790,734 $4,660,285 $4,658,287