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Investment Securities, Equity Securities with Readily Determinable Fair Values and Other Investments (Tables) | 12 Months Ended
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Dec. 31, 2022
Amortized cost and estimated fair value by type of investment security | 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 Debt Securities | ​
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Amortized | ​ | unrealized | ​ | unrealized | ​ | Estimated | ​ | Carrying | ​
​ | ​ | cost | ​ | gains | ​ | losses | ​ | fair 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 Mac
The amortized cost and estimated fair value by type of investment security at December 31, 2021 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) | ​
Residential mortgage-backed securities |  | $ | 4,213,441 |  | $ | 14,159 |  | $ | (58,237) |  | $ | 4,169,363 |  | $ | 4,169,363 | ​
Obligations of states and political subdivisions | ​ |  | 41,519 | ​ |  | 3,038 | ​ |  | — | ​ |  | 44,557 | ​ |  | 44,557 | ​
Total investment securities | ​ | $ | 4,254,960 | ​ | $ | 17,197 | ​ | $ | (58,237) | ​ | $ | 4,213,920 | ​ | $ | 4,213,920 | ​
(1) | Included in the carrying value of residential mortgage- backed securities are $824,474 of mortgage-backed securities issued by Ginnie Mae, $3,344,889 of mortgage-backed securities issued by Fannie Mae and Freddie Mac
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 |  | $ | 1,200 |  | $ | 1,200 |  | $ | 49,752 |  | $ | 49,393 | ​
Due after one year through five years | ​ |  | 2,200 | ​ |  | 2,200 | ​ |  | — | ​ | ​ | — | ​
Due after ten years | ​ |  | — | ​ |  | — | ​ |  | 163,509 | ​ |  | 159,191 | ​
Residential mortgage-backed securities | ​ |  | — | ​ |  | — | ​ |  | 4,805,735 | ​ |  | 4,209,212 | ​
Total investment securities | ​ | $ | 3,400 | ​ | $ | 3,400 | ​ | $ | 5,018,996 | ​ | $ | 4,417,796 | ​
​
Gross unrealized losses on investment securities and the related fair value | Gross unrealized losses on investment securities and the fair value of the related securities, aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position, at December 31, 2022 were as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Less than 12 months | ​ | 12 months or more | ​ | Total | ​
​ | ​ | ​ | ​ | ​ | Unrealized | ​ | ​ | ​ | ​ | Unrealized | ​ | ​ | ​ | ​ | Unrealized | ​
​ | ​ | Fair Value | ​ | Losses | ​ | Fair Value | ​ | Losses | ​ | Fair Value | ​ | Losses | ​
​ | ​ | (Dollars in Thousands) | ​
Available for sale: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
U.S. Treasury securities | ​ | $ | 49,394 | ​ | $ | (359) | ​ | $ | — | ​ | $ | — | ​ | $ | 49,394 | ​ | $ | (359) | ​
Residential mortgage-backed securities |  | ​ | 1,357,905 |  | ​ | (87,815) |  | ​ | 2,566,975 |  | ​ | (511,853) |  | ​ | 3,924,880 |  | ​ | (599,668) | ​
Obligations of states and political subdivisions | ​ |  | 118,772 | ​ |  | (5,245) | ​ |  | — | ​ |  | — | ​ |  | 118,772 | ​ |  | (5,245) | ​
​ | ​ | $ | 1,526,071 | ​ | $ | (93,419) | ​ | $ | 2,566,975 | ​ | $ | (511,853) | ​ | $ | 4,093,046 | ​ | $ | (605,272) | ​
​ Gross unrealized losses on investment securities and the fair value of the related securities, aggregated by investment category and length of time that individual securities have been in a continuous loss position, at December 31, 2021 were as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Less than 12 months | ​ | 12 months or more | ​ | Total | ​
​ | ​ | ​ | ​ | ​ | Unrealized | ​ | ​ | ​ | ​ | Unrealized | ​ | ​ | ​ | ​ | Unrealized | ​
​ | ​ | Fair Value | ​ | Losses | ​ | Fair Value | ​ | Losses | ​ | Fair Value | ​ | Losses | ​
​ | ​ | (Dollars in Thousands) | ​
Available for sale: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Residential mortgage-backed securities |  | $ | 3,037,188 |  | $ | (53,060) |  | $ | 423,733 |  | $ | (5,177) |  | $ | 3,460,921 |  | $ | (58,237) | ​
​ | ​ | $ | 3,037,188 | ​ | $ | (53,060) | ​ | $ | 423,733 | ​ | $ | (5,177) | ​ | $ | 3,460,921 | ​ | $ | (58,237) | ​
​
Summary of unrealized and realized gains and losses recognized in net income on equity securities | ​
​ | ​ | ​ | ​ | ​
​ | ​ | Year Ended
​ | ​ | December 31, 2022
​ | ​ | (Dollars in Thousands)
​ | ​ | ​
Net losses recognized during the period on equity securities |  | $ | ​ | (721)
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 | ​ | $ | ​ | (721)
​
​ | ​ | ​ | ​ | ​
​ | ​ | Year Ended
​ | ​ | December 31, 2021
​ | ​ | (Dollars in Thousands)
​ | ​ | ​
Net losses recognized during the period on equity securities |  | $ | ​ | (123)
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 | ​ | $ | ​ | (123)
​ ​ ​
​ | ​ | ​ | ​ | ​
​ | ​ | Year Ended
​ | ​ | December 31, 2020
​ | ​ | (Dollars in Thousands)
​ | ​ | ​
Net gains recognized during the period on equity securities |  | $ | ​ | 107
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 | ​ | $ | ​ | 107
​

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