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Investment Securities (Tables)
 3 Months Ended
Mar. 31, 2022
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
AFS investment securities:
March 31, 2022    
U.S. Treasury$14,930 $— $(940)$13,990 
Agency406,654 — (30,877)375,777 
Corporate debt612,647 702 (18,284)595,065 
Municipal bonds445,537 58 (30,311)415,284 
Collateralized mortgage obligations874,382 1,606 (20,732)855,256 
Mortgage-backed securities1,040,774 1 (74,052)966,723 
Total AFS investment securities$3,394,924 $2,367 $(175,196)$3,222,095 
December 31, 2021
U.S. Treasury$57,708 $614 $(456)$57,866 
Agency440,183 2,081 (10,129)432,135 
Corporate debt451,621 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 AFS investment securities$4,278,589 $46,232 $(50,957)$4,273,864 

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
HTM investment securities:
March 31, 2022
Municipal bonds$747,887 $(41)$747,846 $— $(101,757)$646,089 
Mortgage-backed securities247,050 — 247,050 96 (8,671)238,475 
Other1,486 — 1,486 — — 1,486 
Total HTM investment securities$996,423 $(41)$996,382 $96 $(110,428)$886,050 
December 31, 2021
Municipal bonds$368,344 $(22)$368,322 $3,834 $(1,649)$370,507 
Mortgage-backed securities$11,843 $— $11,843 $564 $— $12,407 
Other1,509 — 1,509 — — 1,509 
Total HTM investment securities$381,696 $(22)$381,674 $4,398 $(1,649)$384,423 
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, 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. Treasury1 $13,990 $(940)— $— $— 1 $13,990 $(940)
Agency9 72,300 (6,489)26 303,476 (24,388)35 375,776 (30,877)
Corporate debt44 449,423 (10,400)3 52,241 (7,884)47 501,664 (18,284)
Municipal bonds85 397,687 (30,158)1 1,444 (153)86 399,131 (30,311)
Collateralized mortgage obligations32 320,962 (10,317)23 165,246 (10,415)55 486,208 (20,732)
Mortgage-backed securities.71 880,417 (66,282)9 86,208 (7,770)80 966,625 (74,052)
Total AFS investment securities242 $2,134,779 $(124,586)62 $608,615 $(50,610)304 $2,743,394 $(175,196)
 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
AFS investment securities:
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)
Corporate debt17 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 AFS investment securities204 $2,302,658 $(35,173)51 $477,926 $(15,784)255 $2,780,584 $(50,957)
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, 2022
(Dollars in thousands)
 Balance,
December 31, 2021
Provision for Credit Losses
Balance,
March 31, 2022
HTM investment securities:
Municipal bonds$22 $19 $41 
Schedule of investment securities by external credit rating The following table summarizes the Company’s investment securities portfolio by Moody’s external rating equivalent and by vintage as of March 31, 2022:
Vintage
(Dollars in thousands)20222021202020192018PriorTotal
AFS Investment securities at fair value:
U.S. Treasury
Aaa - Aa3$— $13,990 $— $— $— $— $13,990 
Agency
Aaa - Aa3— 22,827 282,195 23,967 33,635 6,603 369,227 
Baa1 - Baa3— — — — — 6,550 6,550 
Corporate debt
Aaa - Aa339,946 — — — — — 39,946 
A1 - A374,565 203,779 62,222 — — — 340,566 
Baa1 - Baa334,267 52,711 92,122 25,664 — 9,789 214,553 
Municipal bonds
Aaa - Aa3— 43,333 222,497 136,748 — 12,706 415,284 
Collateralized mortgage obligations
Aaa - Aa3173,442 260,990 229,588 74,720 14,839 101,677 855,256 
Mortgage-backed securities
Aaa - Aa319,610 772,190 113,748 58,778 — 2,397 966,723 
Total AFS investment securities341,830 1,369,820 1,002,372 319,877 48,474 139,722 3,222,095 
HTM investment securities at amortized cost:
Municipal bonds
Aaa - Aa3— 116,523 622,498 8,866 — — 747,887 
Mortgage-backed securities
Aaa - Aa3— 135,396 101,116 — 2,942 7,596 247,050 
Other
Baa1 - Baa3— — — — 582 904 1,486 
Total HTM investment securities— 251,919 723,614 8,866 3,524 8,500 996,423 
Total investment securities$341,830 $1,621,739 $1,725,986 $328,743 $51,998 $148,222 $4,218,518 
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,December 31,March 31,
(Dollars in thousands)202220212021
Amortized cost of AFS investment securities sold$658,505 $267,114 $172,320 
Gross realized gains$13,637 $4,723 $4,237 
Gross realized losses(11,503)(1,138)(191)
Net realized gains on sales of AFS investment securities$2,134 $3,585 $4,046 
Schedule of amortized cost and estimated fair value of investment securities available for sale by contractual maturity
The amortized cost and estimated fair value of investment securities at March 31, 2022, by contractual maturity, are shown in the table below.
Due in One Year
or Less		Due after One Year
through Five Years		Due after Five Years
through Ten Years		Due after
Ten Years		Total
(Dollars in thousands)Amortized
Cost		Fair
Value		Amortized
Cost		Fair
Value		Amortized
Cost		Fair
Value		Amortized
Cost		Fair
Value		Amortized
Cost		Fair
Value
AFS investment securities:          
U.S. Treasury$— $— $— $— $14,930 $13,990 $— $— $14,930 $13,990 
Agency— — 271,342 254,013 97,419 88,539 37,893 33,225 406,654 375,777 
Corporate debt— — 192,589 190,794 405,058 389,354 15,000 14,917 612,647 595,065 
Municipal bonds— — — — 18,061 16,806 427,476 398,478 445,537 415,284 
Collateralized mortgage obligations— — 36,738 36,544 243,674 231,495 593,970 587,217 874,382 855,256 
Mortgage-backed securities— — — — 612,287 572,033 428,487 394,690 1,040,774 966,723 
Total AFS investment securities— — 500,669 481,351 1,391,429 1,312,217 1,502,826 1,428,527 3,394,924 3,222,095 
HTM investment securities:
Municipal bonds— — — — 44,774 42,509 703,113 603,580 747,887 646,089 
Mortgage-backed securities— — — — — — 247,050 238,475 247,050 238,475 
Other— — — — — — 1,486 1,486 1,486 1,486 
Total HTM investment securities— — — — 44,774 42,509 951,649 843,541 996,423 886,050 
Total investment securities$— $— $500,669 $481,351 $1,436,203 $1,354,726 $2,454,475 $2,272,068 $4,391,347 $4,108,145