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Investment Securities and Equity Securities with Readily Determinable Fair Values (Tables) | 6 Months Ended
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Jun. 30, 2019
Investment Securities |
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, 2019 are as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Held to Maturity | ​
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Amortized | ​ | unrealized | ​ | unrealized | ​ | Estimated | ​ | Carrying | ​
​ | ​ | cost | ​ | gains | ​ | losses | ​ | fair value | ​ | value | ​
​ | ​ | (Dollars in Thousands) | ​
Other securities |  | $ | 2,400 |  | $ | — |  | $ | — |  | $ | 2,400 |  | $ | 2,400 | ​
Total investment securities | ​ | $ | 2,400 | ​ | $ | — | ​ | $ | — | ​ | $ | 2,400 | ​ | $ | 2,400 | ​
​ ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | 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 |  | $ | 3,348,834 |  | $ | 15,432 |  | $ | (24,046) |  | $ | 3,340,220 |  | $ | 3,340,220 | ​
Obligations of states and political subdivisions | ​ |  | 124,646 | ​ |  | 3,047 | ​ |  | — | ​ |  | 127,693 | ​ |  | 127,693 | ​
Total investment securities | ​ | $ | 3,473,480 | ​ | $ | 18,479 | ​ | $ | (24,046) | ​ | $ | 3,467,913 | ​ | $ | 3,467,913 | ​
(1) | Included in the carrying value of residential mortgage-backed securities are $544,659 of mortgage-backed securities issued by Ginnie Mae and $2,795,561 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, 2018 are as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Held to Maturity | ​
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Amortized | ​ | unrealized | ​ | unrealized | ​ | Estimated | ​ | Carrying | ​
​ | ​ | cost | ​ | gains | ​ | losses | ​ | fair value | ​ | value | ​
​ | ​ | (Dollars in Thousands) | ​
Other securities |  | $ | 1,200 |  | $ | — |  | $ | — |  | $ | 1,200 |  | $ | 1,200 | ​
Total investment securities | ​ | $ | 1,200 | ​ | $ | — | ​ | $ | — | ​ | $ | 1,200 | ​ | $ | 1,200 | ​
​ ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Available for Sale | ​
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | Estimated | ​ | ​ | ​ | ​
​ | ​ | Amortized | ​ | unrealized | ​ | unrealized | ​ | fair | ​ | Carrying | ​
​ | ​ | cost | ​ | gains | ​ | losses | ​ | value | ​ | value(1) | ​
​ | ​ | (Dollars in Thousands) | ​
Residential mortgage-backed securities |  | $ | 3,295,366 |  | $ | 6,813 |  | $ | (79,169) |  | $ | 3,223,010 |  | $ | 3,223,010 | ​
Obligations of states and political subdivisions | ​ |  | 185,799 | ​ |  | 2,646 | ​ |  | (105) | ​ |  | 188,340 | ​ |  | 188,340 | ​
Total investment securities | ​ | $ | 3,481,165 | ​ | $ | 9,459 | ​ | $ | (79,274) | ​ | $ | 3,411,350 | ​ | $ | 3,411,350 | ​
(1) | Included in the carrying value of residential mortgage-backed securities are $501,293 of mortgage-backed securities issued by Ginnie Mae and $2,721,717 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 |  | $ | — |  | $ | — | ​
Due after one year through five years | ​ |  | 1,325 | ​ |  | 1,325 | ​ |  | — | ​ |  | — | ​
Due after five years through ten years | ​ |  | — | ​ |  | — | ​ |  | 2,244 | ​ |  | 2,283 | ​
Due after ten years | ​ |  | — | ​ |  | — | ​ |  | 122,402 | ​ |  | 125,410 | ​
Residential mortgage-backed securities | ​ |  | — | ​ |  | — | ​ |  | 3,348,834 | ​ |  | 3,340,220 | ​
Total investment securities | ​ | $ | 2,400 | ​ | $ | 2,400 | ​ | $ | 3,473,480 | ​ | $ | 3,467,913 | ​
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, 2019, 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 |  | $ | 23,726 |  | $ | (72) |  | $ | 2,080,460 |  | $ | (23,974) |  | $ | 2,104,186 |  | $ | (24,046) | ​
​ | ​ | $ | 23,726 | ​ | $ | (72) | ​ | $ | 2,080,460 | ​ | $ | (23,974) | ​ | $ | 2,104,186 | ​ | $ | (24,046) | ​
​ 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, 2018 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 |  | $ | 208,384 |  | $ | (2,124) |  | $ | 2,537,181 |  | $ | (77,045) |  | $ | 2,745,565 |  | $ | (79,169) | ​
Obligations of states and political subdivisions | ​ |  | 12,756 | ​ |  | (99) | ​ |  | 512 | ​ |  | (6) | ​ |  | 13,268 | ​ |  | (105) | ​
​ | ​ | $ | 221,140 | ​ | $ | (2,223) | ​ | $ | 2,537,693 | ​ | $ | (77,051) | ​ | $ | 2,758,833 | ​ | $ | (79,274) | ​
​
Summary of unrealized and realized gains and losses recognized in net income on equity securities | ​
​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended
​ | ​ | June 30, 2019
​ | ​ | (Dollars in Thousands)
​ | ​ | ​
​ | ​ | ​ | ​ | ​
Net gains (losses) recognized during the period on equity securities |  | $ | ​ | 52
Less: Net gains and (losses) recognized during the period on equity securities sold during the period | ​ |  | ​ | —
​ | ​ | ​ | ​ | ​
Unrealized gains (losses) recognized during the reporting period on equity securities still held at the reporting date | ​ | $ | ​ | 52
​
​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended
​ | ​ | June 30, 2018
​ | ​ | (Dollars in Thousands)
​ | ​ | ​
​ | ​ | ​ | ​ | ​
Net gains (losses) recognized during the period on equity securities |  | $ | ​ | (247)
Less: Net gains and (losses) recognized during the period on equity securities sold during the period | ​ |  | ​ | —
​ | ​ | ​ | ​ | ​
Unrealized gains (losses) recognized during the reporting period on equity securities still held at the reporting date | ​ | $ | ​ | (247)
​
​ | ​ | ​ | ​ | ​
​ | ​ | Six Months Ended
​ | ​ | June 30, 2019
​ | ​ | (Dollars in Thousands)
​ | ​ | ​
Net gains (losses) recognized during the period on equity securities |  | $ | ​ | 190
Less: Net gains and (losses) recognized during the period on equity securities sold during the period | ​ |  | ​ | —
​ | ​ | ​ | ​ | ​
Unrealized gains (losses) recognized during the reporting period on equity securities still held at the reporting date | ​ | $ | ​ | 190
​
​ | ​ | ​ | ​ | ​
​ | ​ | Six Months Ended
​ | ​ | June 30, 2018
​ | ​ | (Dollars in Thousands)
​ | ​ | ​
Net gains (losses) recognized during the period on equity securities |  | $ | ​ | (1,021)
Less: Net gains and (losses) recognized during the period on equity securities sold during the period | ​ |  | ​ | —
​ | ​ | ​ | ​ | ​
Unrealized gains (losses) recognized during the reporting period on equity securities still held at the reporting date | ​ | $ | ​ | (1,021)
​

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