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Investment Securities, Equity Securities with Readily Determinable Fair Values and Other Investments (Tables) | 3 Months Ended
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Mar. 31, 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 March 31, 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) | ​
U.S. Treasury securities |  | $ | 49,967 |  | $ | — |  | $ | (42) |  | $ | 49,925 |  | $ | 49,925 | ​
Residential mortgage-backed securities | ​ | ​ | 5,184,217 | ​ | ​ | 5,289 | ​ | ​ | (542,277) | ​ | ​ | 4,647,229 | ​ | ​ | 4,647,229 | ​
Obligations of states and political subdivisions | ​ |  | 162,947 | ​ |  | 1,485 | ​ |  | (1,768) | ​ |  | 162,664 | ​ |  | 162,664 | ​
Total investment securities | ​ | $ | 5,397,131 | ​ | $ | 6,774 | ​ | $ | (544,087) | ​ | $ | 4,859,818 | ​ | $ | 4,859,818 | ​
(1) | Included in the carrying value of residential mortgage-backed securities are $810,907 of mortgage-backed securities issued by Ginnie Mae and $3,836,322 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 |  | $ | 1,075 |  | $ | 1,075 |  | $ | 49,967 |  | $ | 49,925 | ​
Due after one year through five years | ​ |  | 2,325 | ​ |  | 2,325 | ​ |  | — | ​ | ​ | — | ​
Due after five years through ten years | ​ |  | — | ​ |  | — | ​ |  | 440 | ​ |  | 440 | ​
Due after ten years | ​ |  | — | ​ |  | — | ​ |  | 162,507 | ​ |  | 162,224 | ​
Residential mortgage-backed securities | ​ |  | — | ​ |  | — | ​ |  | 5,184,217 | ​ |  | 4,647,229 | ​
Total investment securities | ​ | $ | 3,400 | ​ | $ | 3,400 | ​ | $ | 5,397,131 | ​ | $ | 4,859,818 | ​
​
Gross unrealized losses on investment securities and the related fair value | ​ Gross unrealized losses on debt investment securities and the fair value of the related securities, aggregated by investment category and length of time that individual debt securities have been in a continuous unrealized loss position at March 31, 2023, 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 | ​ | $ | 49,925 | ​ | $ | (42) | ​ | $ | — | ​ | $ | — | ​ | $ | 49,925 | ​ | $ | (42) | ​
Residential mortgage-backed securities |  | ​ | 1,438,275 |  | ​ | (44,176) |  | ​ | 2,969,348 |  | ​ | (498,101) |  | ​ | 4,407,623 |  | ​ | (542,277) | ​
Obligations of states and political subdivisions | ​ |  | 105,758 | ​ |  | (1,768) | ​ |  | — | ​ |  | — | ​ |  | 105,758 | ​ |  | (1,768) | ​
​ | ​ | $ | 1,593,958 | ​ | $ | (45,986) | ​ | $ | 2,969,348 | ​ | $ | (498,101) | ​ | $ | 4,563,306 | ​ | $ | (544,087) | ​
​ 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)
​
Summary of unrealized and realized gains and losses recognized in net income on equity securities | ​
​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended
​ | ​ | March 31, 2023
​ | ​ | (Dollars in Thousands)
​ | ​ | ​
​ | ​ | ​ | ​ | ​
Net gains recognized during the period on equity securities |  | $ | ​ | 34
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 | ​ | $ | ​ | 34
​ ​ ​
​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended
​ | ​ | March 31, 2022
​ | ​ | (Dollars in Thousands)
​ | ​ | ​
​ | ​ | ​ | ​ | ​
Net losses recognized during the period on equity securities |  | $ | ​ | (269)
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 | ​ | $ | ​ | (269)
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

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