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Investment Securities, Equity Securities with Readily Determinable Fair Values and Other Investments (Tables) | 6 Months Ended
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Jun. 30, 2022
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 June 30, 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 |  | $ | 49,314 |  | $ | — |  | $ | (363) |  | $ | 48,951 |  | $ | 48,951 | ​
Residential mortgage-backed securities | ​ | ​ | 4,557,327 | ​ | ​ | 423 | ​ | ​ | (353,819) | ​ | ​ | 4,203,931 | ​ | ​ | 4,203,931 | ​
Obligations of states and political subdivisions | ​ |  | 41,390 | ​ |  | 261 | ​ |  | (173) | ​ |  | 41,478 | ​ |  | 41,478 | ​
Total investment securities | ​ | $ | 4,648,031 | ​ | $ | 684 | ​ | $ | (354,355) | ​ | $ | 4,294,360 | ​ | $ | 4,294,360 | ​
(1) | Included in the carrying value of residential mortgage-backed securities are $680,229 of mortgage-backed securities issued by Ginnie Mae and $3,523,702 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 and $3,344,899 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 |  | $ | 1,325 |  | $ | 1,325 |  | $ | — |  | $ | — | ​
Due after one year through five years | ​ |  | 2,075 | ​ |  | 2,075 | ​ |  | 49,314 | ​ | ​ | 48,951 | ​
Due after ten years | ​ |  | — | ​ |  | — | ​ |  | 41,390 | ​ |  | 41,478 | ​
Residential mortgage-backed securities | ​ |  | — | ​ |  | — | ​ |  | 4,557,327 | ​ |  | 4,203,931 | ​
Total investment securities | ​ | $ | 3,400 | ​ | $ | 3,400 | ​ | $ | 4,648,031 | ​ | $ | 4,294,360 | ​
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 June 30, 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 | ​ | $ | 48,951 | ​ | $ | (363) | ​ | $ | — | ​ | $ | — | ​ | $ | 48,951 | ​ | $ | (363) | ​
Residential mortgage-backed securities |  | ​ | 2,892,440 |  | ​ | (254,453) |  | ​ | 1,029,269 |  | ​ | (99,366) |  | ​ | 3,921,709 |  | ​ | (353,819) | ​
Obligations of states and political subdivisions | ​ |  | 19,489 | ​ |  | (173) | ​ |  | — | ​ |  | — | ​ |  | 19,489 | ​ |  | (173) | ​
​ | ​ | $ | 2,960,880 | ​ | $ | (254,989) | ​ | $ | 1,029,269 | ​ | $ | (99,366) | ​ | $ | 3,990,149 | ​ | $ | (354,355) | ​
​ 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, 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 | ​
​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended
​ | ​ | June 30, 2022
​ | ​ | (Dollars in Thousands)
​ | ​ | ​
​ | ​ | ​ | ​ | ​
Net losses recognized during the period on equity securities |  | $ | ​ | (229)
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 | ​ | $ | ​ | (229)
​
​ | ​ | ​ | ​ | ​
​ | ​ | Six Months Ended
​ | ​ | June 30, 2022
​ | ​ | (Dollars in Thousands)
​ | ​ | ​
Net losses recognized during the period on equity securities |  | $ | ​ | (498)
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 | ​ | $ | ​ | (498)
​ ​
​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended
​ | ​ | June 30, 2021
​ | ​ | (Dollars in Thousands)
​ | ​ | ​
​ | ​ | ​ | ​ | ​
Net gains recognized during the period on equity securities |  | $ | ​ | 11
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 | ​ | $ | ​ | 11
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ | ​ | ​ | ​ | ​
​ | ​ | Six Months Ended
​ | ​ | June 30, 2021
​ | ​ | (Dollars in Thousands)
​ | ​ | ​
Net losses recognized during the period on equity securities |  | $ | ​ | (65)
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 | ​ | $ | ​ | (65)

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