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SECURITIES (Tables) | 9 Months Ended
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Sep. 30, 2023
Amortized Cost of HTM Securities by Security Type and Credit Rating | The following table presents the amortized cost of HTM securities as of September 30, 2023 and December 31, 2022 by security type and credit rating (dollars in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | U.S. Government and Agency |  | Obligations of states and political |  | Corporate and other |  | Mortgage-backed |  | Total HTM
​ | ​ | securities | ​ | subdivisions | ​ | bonds | ​ | securities | ​ | securities
September 30, 2023 | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Credit Rating: | ​ |  | ​ | ​ |  | ​ | ​ |  | ​ | ​ | ​ | ​ | ​ | ​ | ​
AAA/AA/A | ​ | $ | — | ​ | $ | 689,699 | ​ | $ | — | ​ | $ | 9,471 | ​ | $ | 699,170
BBB/BB/B | ​ | ​ | — | ​ | ​ | 1,171 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 1,171
Not Rated – Agency(1) | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 68,561 | ​ | ​ | 68,561
Not Rated – Non-Agency(2) | ​ | ​ | — | ​ |  | 9,530 | ​ |  | 4,536 | ​ | ​ | 60,301 | ​ | ​ | 74,367
Total | ​ | $ | — | ​ | $ | 700,400 | ​ | $ | 4,536 | ​ | $ | 138,333 | ​ | $ | 843,269
December 31, 2022 | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Credit Rating: | ​ |  | ​ | ​ |  | ​ | ​ |  | ​ | ​ | ​ | ​ | ​ | ​ | ​
AAA/AA/A | ​ | $ | — | ​ | $ | 704,803 | ​ | $ | — | ​ | $ | 2,702 | ​ | $ | 707,505
BBB/BB/B | ​ | ​ | — | ​ | ​ | 1,187 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 1,187
Not Rated – Agency(1) | ​ | ​ | 687 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 71,725 | ​ | ​ | 72,412
Not Rated – Non-Agency(2) | ​ | ​ | — | ​ |  | — | ​ |  | 5,159 | ​ | ​ | 61,469 | ​ | ​ | 66,628
Total | ​ | $ | 687 | ​ | $ | 705,990 | ​ | $ | 5,159 | ​ | $ | 135,896 | ​ | $ | 847,732
(1) Generally considered not to have credit risk given the government guarantees associated with these agencies. (2) Non-agency mortgage-backed and asset-backed securities have limited credit risk, supported by most receiving a 20% simplified supervisory formula approach rating.
Gross Realized Gain and Losses on the Sale of Securities | The following table presents the gross realized gains and losses on and the proceeds from the sale of securities during the three and nine months ended September 30, 2023 and 2022 (dollars in thousands):
​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | Three Months Ended |  | Nine Months Ended
​ | ​ | September 30, 2023 | ​ | September 30, 2023
Realized gains (losses)(1): |  | ​ |  |  | ​ |
Gross realized gains | ​ | $ | 4 | ​ | $ | 1,352
Gross realized losses | ​ |  | (27,598) | ​ |  | (42,344)
Net realized losses | ​ | $ | (27,594) | ​ | $ | (40,992)
​ | ​ | ​ | ​ | ​ | ​ | ​
Proceeds from sales of securities | ​ | $ | 256,780 | ​ | $ | 856,881
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | Three Months Ended |  | Nine Months Ended
​ | ​ | September 30, 2022 | ​ | September 30, 2022
Realized losses(1): |  | ​ |  |  | ​ |
Gross realized losses | ​ |  | (2) | ​ |  | (2)
Net realized losses | ​ | $ | (2) | ​ | $ | (2)
​ | ​ | ​ | ​ | ​ | ​ | ​
Proceeds from sales of securities | ​ | $ | 12,469 | ​ | $ | 12,469
(1) Includes gains (losses) on sales and calls of securities.
Available-for-sale Securities |
Amortized Cost, Gross Unrealized Gains and Losses, and Estimated Fair Values of Investment Securities | The amortized cost, gross unrealized gains and losses, and estimated fair values of AFS securities as of September 30, 2023 are summarized as follows (dollars in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Amortized | ​ | Gross Unrealized | ​ | Estimated
​ |  | Cost |  | Gains |  | (Losses) |  | Fair Value
September 30, 2023 |  | ​ |  |  | ​ |  |  | ​ |  | ​ | ​ |
U.S. government and agency securities | ​ | $ | 62,221 | ​ | $ | — | ​ | $ | (383) | ​ | $ | 61,838
Obligations of states and political subdivisions | ​ |  | 587,340 | ​ |  | — | ​ |  | (169,880) | ​ |  | 417,460
Corporate and other bonds (1) | ​ |  | 267,816 | ​ |  | — | ​ |  | (24,998) | ​ |  | 242,818
Commercial MBS | ​ |  | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ |  | ​
Agency | ​ | ​ | 215,203 | ​ |  | 307 | ​ |  | (50,289) | ​ | ​ | 165,221
Non-agency | ​ | ​ | 72,057 | ​ |  | — | ​ |  | (2,575) | ​ | ​ | 69,482
Total commercial MBS | ​ | ​ | 287,260 | ​ |  | 307 | ​ |  | (52,864) | ​ | ​ | 234,703
Residential MBS | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Agency | ​ | ​ | 1,319,987 | ​ |  | 1 | ​ |  | (268,152) | ​ | ​ | 1,051,836
Non-agency | ​ | ​ | 81,677 | ​ |  | — | ​ |  | (7,131) | ​ | ​ | 74,546
Total residential MBS | ​ | ​ | 1,401,664 | ​ |  | 1 | ​ |  | (275,283) | ​ | ​ | 1,126,382
Other securities | ​ |  | 1,727 | ​ |  | — | ​ |  | — | ​ |  | 1,727
Total AFS securities | ​ | $ | 2,608,028 | ​ | $ | 308 | ​ | $ | (523,408) | ​ | $ | 2,084,928
(1) Other bonds include asset-backed securities. The amortized cost, gross unrealized gains and losses, and estimated fair values of AFS securities as of December 31, 2022 are summarized as follows (dollars in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Amortized | ​ | Gross Unrealized | ​ | Estimated
​ |  | Cost |  | Gains |  | (Losses) |  | Fair Value
December 31, 2022 | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
U.S. government and agency securities | ​ | $ | 70,196 | ​ | $ | — | ​ | $ | (8,253) | ​ | $ | 61,943
Obligations of states and political subdivisions | ​ | ​ | 959,999 | ​ |  | 137 | ​ |  | (152,701) | ​ |  | 807,435
Corporate and other bonds (1) | ​ |  | 243,979 | ​ |  | — | ​ |  | (17,599) | ​ |  | 226,380
Commercial MBS | ​ |  | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ |  | ​
Agency | ​ | ​ | 250,186 | ​ |  | 75 | ​ |  | (39,268) | ​ | ​ | 210,993
Non-agency | ​ | ​ | 99,412 | ​ |  | — | ​ |  | (4,244) | ​ | ​ | 95,168
Total commercial MBS | ​ | ​ | 349,598 | ​ |  | 75 | ​ |  | (43,512) | ​ | ​ | 306,161
Residential MBS | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Agency | ​ | ​ | 1,510,110 | ​ |  | 81 | ​ |  | (233,961) | ​ | ​ | 1,276,230
Non-agency | ​ | ​ | 68,815 | ​ |  | — | ​ |  | (6,812) | ​ | ​ | 62,003
Total residential MBS | ​ | ​ | 1,578,925 | ​ |  | 81 | ​ |  | (240,773) | ​ | ​ | 1,338,233
Other securities | ​ |  | 1,664 | ​ |  | — | ​ |  | — | ​ |  | 1,664
Total AFS securities | ​ | $ | 3,204,361 | ​ | $ | 293 | ​ | $ | (462,838) | ​ | $ | 2,741,816
(1) Other bonds include asset-backed securities.
Schedule of Gross Unrealized Losses and Fair Value of Investments | The following table shows the gross unrealized losses and fair value of the Company’s AFS securities with unrealized losses. These are aggregated by investment category and length of time that the individual securities have been in a continuous unrealized loss position (dollars in thousands). ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Less than 12 months | ​ | More than 12 months | ​ | Total
​ |  | Fair |  | Unrealized |  | Fair |  | Unrealized |  | Fair |  | Unrealized
​ | ​ | Value | ​ | Losses | ​ | Value(2) | ​ | Losses | ​ | Value | ​ | Losses
September 30, 2023 | ​ |  | ​ | ​ |  | ​ | ​ |  | ​ | ​ |  | ​ | ​ |  | ​ | ​ |  | ​
U.S. government and agency securities | ​ | $ | 59,568 | ​ | $ | (349) | ​ | $ | 2,169 | ​ | $ | (34) | ​ | $ | 61,737 | ​ | $ | (383)
Obligations of states and political subdivisions | ​ | ​ | 16,023 | ​ | ​ | (3,567) | ​ | ​ | 399,438 | ​ | ​ | (166,313) | ​ | ​ | 415,461 | ​ | ​ | (169,880)
Corporate and other bonds(1) | ​ |  | 100,231 | ​ |  | (776) | ​ |  | 142,587 | ​ |  | (24,222) | ​ |  | 242,818 | ​ |  | (24,998)
Commercial MBS | ​ |  | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Agency | ​ | ​ | 15,074 | ​ | ​ | (709) | ​ | ​ | 135,725 | ​ | ​ | (49,580) | ​ | ​ | 150,799 | ​ | ​ | (50,289)
Non-agency | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 69,482 | ​ | ​ | (2,575) | ​ | ​ | 69,482 | ​ | ​ | (2,575)
Total commercial MBS | ​ | ​ | 15,074 | ​ | ​ | (709) | ​ | ​ | 205,207 | ​ | ​ | (52,155) | ​ | ​ | 220,281 | ​ | ​ | (52,864)
Residential MBS | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Agency | ​ | ​ | 55,550 | ​ | ​ | (461) | ​ | ​ | 972,755 | ​ | ​ | (267,691) | ​ | ​ | 1,028,305 | ​ | ​ | (268,152)
Non-agency | ​ | ​ | 19,776 | ​ | ​ | (90) | ​ | ​ | 45,745 | ​ | ​ | (7,041) | ​ | ​ | 65,521 | ​ | ​ | (7,131)
Total residential MBS | ​ | ​ | 75,326 | ​ | ​ | (551) | ​ | ​ | 1,018,500 | ​ | ​ | (274,732) | ​ | ​ | 1,093,826 | ​ | ​ | (275,283)
Total AFS securities | ​ | $ | 266,222 | ​ | $ | (5,952) | ​ | $ | 1,767,901 | ​ | $ | (517,456) | ​ | $ | 2,034,123 | ​ | $ | (523,408)
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
December 31, 2022 | ​ |  |  | ​ |  |  | ​ |  |  | ​ |  |  | ​ |  |  | ​ |  |
U.S. government and agency securities | ​ | $ | 2,594 | ​ | $ | (166) | ​ | $ | 59,269 | ​ | $ | (8,087) | ​ | $ | 61,863 | ​ | $ | (8,253)
Obligations of states and political subdivisions | ​ | ​ | 588,668 | ​ | ​ | (86,895) | ​ | ​ | 187,375 | ​ | ​ | (65,806) | ​ | ​ | 776,043 | ​ | ​ | (152,701)
Corporate and other bonds(1) | ​ |  | 206,861 | ​ |  | (15,019) | ​ |  | 17,121 | ​ |  | (2,580) | ​ |  | 223,982 | ​ |  | (17,599)
Commercial MBS | ​ |  | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Agency | ​ | ​ | 73,362 | ​ | ​ | (7,024) | ​ | ​ | 127,193 | ​ | ​ | (32,244) | ​ | ​ | 200,555 | ​ | ​ | (39,268)
Non-agency | ​ | ​ | 66,618 | ​ | ​ | (2,231) | ​ | ​ | 28,550 | ​ | ​ | (2,013) | ​ | ​ | 95,168 | ​ | ​ | (4,244)
Total commercial MBS | ​ | ​ | 139,980 | ​ | ​ | (9,255) | ​ | ​ | 155,743 | ​ | ​ | (34,257) | ​ | ​ | 295,723 | ​ | ​ | (43,512)
Residential MBS | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Agency | ​ | ​ | 328,590 | ​ | ​ | (27,769) | ​ | ​ | 929,581 | ​ | ​ | (206,192) | ​ | ​ | 1,258,171 | ​ | ​ | (233,961)
Non-agency | ​ | ​ | 18,939 | ​ | ​ | (1,288) | ​ | ​ | 43,064 | ​ | ​ | (5,524) | ​ | ​ | 62,003 | ​ | ​ | (6,812)
Total residential MBS | ​ | ​ | 347,529 | ​ | ​ | (29,057) | ​ | ​ | 972,645 | ​ | ​ | (211,716) | ​ | ​ | 1,320,174 | ​ | ​ | (240,773)
Total AFS securities | ​ | $ | 1,285,632 | ​ | $ | (140,392) | ​ | $ | 1,392,153 | ​ | $ | (322,446) | ​ | $ | 2,677,785 | ​ | $ | (462,838)
(1) Other bonds include asset-backed securities. (2) Comprised of 761 and 363 individual securities as of September 30, 2023 and December 31, 2022, respectively. ​
Schedule of Amortized Cost and Estimated Fair Value of Securities | The following table presents the amortized cost and estimated fair value of AFS securities as of September 30, 2023 and December 31, 2022, by contractual maturity (dollars in thousands). Expected maturities may differ from contractual maturities because borrowers may have the right to call or prepay obligations with or without call or prepayment penalties. ​ ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | September 30, 2023 | ​ | December 31, 2022
​ |  | Amortized |  | Estimated |  | Amortized |  | Estimated
​ | ​ | Cost | ​ | Fair Value | ​ | Cost | ​ | Fair Value
Due in one year or less | ​ | $ | 56,948 | ​ | $ | 56,120 | ​ | $ | 42,447 | ​ | $ | 41,735
Due after one year through five years | ​ |  | 138,070 | ​ |  | 135,530 | ​ |  | 158,063 | ​ |  | 152,523
Due after five years through ten years | ​ |  | 262,259 | ​ |  | 236,096 | ​ |  | 343,303 | ​ |  | 312,935
Due after ten years | ​ |  | 2,150,751 | ​ |  | 1,657,182 | ​ |  | 2,660,548 | ​ |  | 2,234,623
Total AFS securities | ​ | $ | 2,608,028 | ​ | $ | 2,084,928 | ​ | $ | 3,204,361 | ​ | $ | 2,741,816
Held-to-maturity Securities |
Schedule of Amortized Cost and Estimated Fair Value of Securities | The following table presents the amortized cost and estimated fair value of HTM securities as of September 30, 2023 and December 31, 2022, by contractual maturity (dollars in thousands). Expected maturities may differ from contractual maturities because borrowers may have the right to call or prepay obligations with or without call or prepayment penalties. ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | September 30, 2023 | ​ | December 31, 2022
​ |  | Carrying |  | Estimated |  | Carrying |  | Estimated
​ | ​ | Value | ​ | Fair Value | ​ | Value | ​ | Fair Value
Due in one year or less | ​ | $ | 2,759 | ​ | $ | 2,735 | ​ | $ | 2,010 | ​ | $ | 2,006
Due after one year through five years | ​ |  | 36,604 | ​ |  | 36,272 | ​ |  | 35,044 | ​ |  | 35,014
Due after five years through ten years | ​ |  | 43,652 | ​ |  | 41,057 | ​ |  | 19,941 | ​ |  | 20,239
Due after ten years | ​ |  | 760,254 | ​ |  | 681,982 | ​ |  | 790,737 | ​ |  | 744,628
Total HTM securities | ​ | $ | 843,269 | ​ | $ | 762,046 | ​ | $ | 847,732 | ​ | $ | 801,887
Schedule of Carrying Values, Gross Unrealized Gains and Losses and Estimated Fair Value of Securities | The carrying value, gross unrealized gains and losses, and estimated fair values of HTM securities as of September 30, 2023 are summarized as follows (dollars in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Carrying | ​ | Gross Unrealized | ​ | Estimated
​ |  | Value |  | Gains |  | (Losses) | ​ | Fair Value
September 30, 2023 |  | ​ |  |  | ​ |  |  | ​ |  | ​ | ​ |
Obligations of states and political subdivisions | ​ | $ | 700,400 | ​ | $ | 49 | ​ | $ | (64,625) | ​ | $ | 635,824
Corporate and other bonds(1) | ​ | ​ | 4,536 | ​ | ​ | — | ​ | ​ | (170) | ​ | ​ | 4,366
Commercial MBS | ​ |  | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Agency | ​ | ​ | 27,568 | ​ | ​ | — | ​ | ​ | (6,788) | ​ | ​ | 20,780
Non-agency | ​ | ​ | 24,930 | ​ | ​ | — | ​ | ​ | (904) | ​ | ​ | 24,026
Total commercial MBS | ​ | ​ | 52,498 | ​ | ​ | — | ​ | ​ | (7,692) | ​ | ​ | 44,806
Residential MBS | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Agency | ​ | ​ | 40,992 | ​ | ​ | — | ​ | ​ | (7,865) | ​ | ​ | 33,127
Non-agency | ​ | ​ | 44,843 | ​ | ​ | — | ​ | ​ | (920) | ​ | ​ | 43,923
Total residential MBS | ​ | ​ | 85,835 | ​ | ​ | — | ​ | ​ | (8,785) | ​ | ​ | 77,050
Total HTM securities | ​ | $ | 843,269 | ​ | $ | 49 | ​ | $ | (81,272) | ​ | $ | 762,046
(1) Other bonds include asset-backed securities. ​ ​ The carrying value, gross unrealized gains and losses, and estimated fair values of HTM securities as of December 31, 2022 are summarized as follows (dollars in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Carrying | ​ | Gross Unrealized | ​ | Estimated
​ |  | Value |  | Gains |  | (Losses) |  | Fair Value
December 31, 2022 | ​ |  |  | ​ |  |  | ​ |  |  | ​ |  |
U.S. government and agency securities | ​ | $ | 687 | ​ | $ | — | ​ | $ | (56) | ​ | $ | 631
Obligations of states and political subdivisions | ​ | ​ | 705,990 | ​ | ​ | 2,218 | ​ | ​ | (35,957) | ​ | ​ | 672,251
Corporate and other bonds(1) | ​ | ​ | 5,159 | ​ | ​ | — | ​ | ​ | (10) | ​ | ​ | 5,149
Commercial MBS | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Agency | ​ | ​ | 29,025 | ​ | ​ | — | ​ | ​ | (4,873) | ​ | ​ | 24,152
Non-agency | ​ | ​ | 13,736 | ​ | ​ | — | ​ | ​ | (126) | ​ | ​ | 13,610
Total commercial MBS | ​ | ​ | 42,761 | ​ | ​ | — | ​ | ​ | (4,999) | ​ | ​ | 37,762
Residential MBS | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Agency | ​ | ​ | 42,699 | ​ | ​ | — | ​ | ​ | (6,427) | ​ | ​ | 36,272
Non-agency | ​ | ​ | 50,436 | ​ | ​ | — | ​ | ​ | (614) | ​ | ​ | 49,822
Total residential MBS | ​ | ​ | 93,135 | ​ | ​ | — | ​ | ​ | (7,041) | ​ | ​ | 86,094
Total HTM securities | ​ | $ | 847,732 | ​ | $ | 2,218 | ​ | $ | (48,063) | ​ | $ | 801,887
(1) Other bonds include asset-backed securities.

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