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Investment Securities, Equity Securities with Readily Determinable Fair Values and Other Investments (Tables) | 9 Months Ended
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Sep. 30, 2023
Investment Securities, Equity Securities with Readily Determinable Fair Values and Other Investments |
Amortized cost and estimated fair value by type of investment security | The amortized cost and estimated fair value by type of investment security at September 30, 2023 are as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Held to Maturity | ​
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Amortized | ​ | unrealized | ​ | unrealized | ​ | Estimated | ​ | Carrying | ​
​ | ​ | cost | ​ | gains | ​ | losses | ​ | fair value | ​ | value | ​
​ | ​ | (Dollars in Thousands) | ​
Other securities |  | $ | 3,400 |  | $ | — |  | $ | — |  | $ | 3,400 |  | $ | 3,400 | ​
Total investment securities | ​ | $ | 3,400 | ​ | $ | — | ​ | $ | — | ​ | $ | 3,400 | ​ | $ | 3,400 | ​
​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Available for Sale Debt Securities | ​
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Amortized | ​ | unrealized | ​ | unrealized | ​ | Estimated | ​ | Carrying | ​
​ | ​ | cost | ​ | gains | ​ | losses | ​ | fair value | ​ | value(1) | ​
​ | ​ | (Dollars in Thousands) | ​
Residential mortgage-backed securities | ​ | $ | 5,227,139 | ​ | $ | — | ​ | $ | (681,539) | ​ | $ | 4,545,600 | ​ | $ | 4,545,600 | ​
Obligations of states and political subdivisions | ​ |  | 161,118 | ​ |  | — | ​ |  | (14,780) | ​ |  | 146,338 | ​ |  | 146,338 | ​
Total investment securities | ​ | $ | 5,388,257 | ​ | $ | — | ​ | $ | (696,319) | ​ | $ | 4,691,938 | ​ | $ | 4,691,938 | ​
(1) | Included in the carrying value of residential mortgage-backed securities are $890,723 of mortgage-backed securities issued by Ginnie Mae and $3,654,877 of mortgage-backed securities issued by Fannie Mae and Freddie Mac.
​ ​ The amortized cost and estimated fair value by type of investment security at December 31, 2022 are as follows:
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Held to Maturity | ​
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Amortized | ​ | unrealized | ​ | unrealized | ​ | Estimated | ​ | Carrying | ​
​ | ​ | cost | ​ | gains | ​ | losses | ​ | fair value | ​ | value | ​
​ | ​ | (Dollars in Thousands) | ​
Other securities |  | $ | 3,400 |  | $ | — |  | $ | — |  | $ | 3,400 |  | $ | 3,400 | ​
Total investment securities | ​ | $ | 3,400 | ​ | $ | — | ​ | $ | — | ​ | $ | 3,400 | ​ | $ | 3,400 | ​
​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Available for Sale | ​
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | Estimated | ​ | ​ | ​ | ​
​ | ​ | Amortized | ​ | unrealized | ​ | unrealized | ​ | fair | ​ | Carrying | ​
​ | ​ | cost | ​ | gains | ​ | losses | ​ | value | ​ | value(1) | ​
​ | ​ | (Dollars in Thousands) | ​
U.S. Treasury securities | ​ | $ | 49,752 | ​ | $ | — |  | $ | (359) | ​ | $ | 49,393 | ​ | $ | 49,393 | ​
Residential mortgage-backed securities |  | ​ | 4,805,735 |  | ​ | 3,145 | ​ | ​ | (599,668) |  | ​ | 4,209,212 |  | ​ | 4,209,212 | ​
Obligations of states and political subdivisions | ​ |  | 163,509 | ​ |  | 927 | ​ |  | (5,245) | ​ |  | 159,191 | ​ |  | 159,191 | ​
Total investment securities | ​ | $ | 5,018,996 | ​ | $ | 4,072 | ​ | $ | (605,272) | ​ | $ | 4,417,796 | ​ | $ | 4,417,796 | ​
(1) | Included in the carrying value of residential mortgage-backed securities are $681,121 of mortgage-backed securities issued by Ginnie Mae and $3,528,091 of mortgage-backed securities issued by Fannie Mae and Freddie.
Amortized cost and fair value of investment securities, by contractual maturity | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Held to Maturity | ​ | Available for Sale | ​
​ | ​ | Amortized | ​ | Estimated | ​ | Amortized | ​ | Estimated | ​
​ | ​ | Cost | ​ | fair value | ​ | Cost | ​ | fair value | ​
​ | ​ | (Dollars in Thousands) | ​
Due in one year or less |  | $ | 2,075 |  | $ | 2,075 |  | $ | — |  | $ | — | ​
Due after one year through five years | ​ |  | 1,325 | ​ |  | 1,325 | ​ |  | — | ​ | ​ | — | ​
Due after five years through ten years | ​ |  | — | ​ |  | — | ​ |  | 440 | ​ |  | 431 | ​
Due after ten years | ​ |  | — | ​ |  | — | ​ |  | 160,678 | ​ |  | 145,907 | ​
Residential mortgage-backed securities | ​ |  | — | ​ |  | — | ​ |  | 5,227,139 | ​ |  | 4,545,600 | ​
Total investment securities | ​ | $ | 3,400 | ​ | $ | 3,400 | ​ | $ | 5,388,257 | ​ | $ | 4,691,938 | ​
Summary of unrealized and realized gains and losses recognized in net income on equity securities | ​
​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended
​ | ​ | September 30, 2023
​ | ​ | (Dollars in Thousands)
​ | ​ | ​
​ | ​ | ​ | ​ | ​
Net losses recognized during the period on equity securities |  | $ | ​ | (142)
Less: Net gains and (losses) recognized during the period on equity securities sold during the period | ​ |  | ​ | —
​ | ​ | ​ | ​ | ​
Unrealized gains recognized during the reporting period on equity securities still held at the reporting date | ​ | $ | ​ | (142)
​
​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended
​ | ​ | September 30, 2022
​ | ​ | (Dollars in Thousands)
​ | ​ | ​
​ | ​ | ​ | ​ | ​
Net losses recognized during the period on equity securities |  | $ | ​ | (230)
Less: Net gains and (losses) recognized during the period on equity securities sold during the period | ​ |  | ​ | —
​ | ​ | ​ | ​ | ​
Unrealized losses recognized during the reporting period on equity securities still held at the reporting date | ​ | $ | ​ | (230)
​ ​ ​ ​ ​ ​ ​ ​ ​
​ | ​ | ​ | ​ | ​
​ | ​ | Nine Months Ended
​ | ​ | September 30, 2023
​ | ​ | (Dollars in Thousands)
​ | ​ | ​
Net losses recognized during the period on equity securities |  | $ | ​ | (200)
Less: Net gains and (losses) recognized during the period on equity securities sold during the period | ​ |  | ​ | —
​ | ​ | ​ | ​ | ​
Unrealized losses recognized during the reporting period on equity securities still held at the reporting date | ​ | $ | ​ | (200)
​ ​ ​
​ | ​ | ​ | ​ | ​
​ | ​ | Nine Months Ended
​ | ​ | September 30, 2022
​ | ​ | (Dollars in Thousands)
​ | ​ | ​
Net losses recognized during the period on equity securities |  | $ | ​ | (728)
Less: Net gains and (losses) recognized during the period on equity securities sold during the period | ​ |  | ​ | —
​ | ​ | ​ | ​ | ​
Unrealized losses recognized during the reporting period on equity securities still held at the reporting date | ​ | $ | ​ | (728)
​

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- Definition Tabular disclosure of realized and unrealized gain (loss) on investment in security. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 220 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03(7)(c)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483621/220-10-S99-2 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 220 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03(9)(a)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483621/220-10-S99-2 + Details
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