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Investment Securities | 12 Months Ended
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Dec. 31, 2019
Investment Securities and Equity Securities with Readily Determinable Fair Values | (2) Investment Securities The amortized cost and estimated fair value by type of investment security at December 31, 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 |  | $ | 3,285,623 |  | $ | 16,534 |  | $ | (16,609) |  | $ | 3,285,548 |  | $ | 3,285,548 | ​
Obligations of states and political subdivisions | ​ |  | 90,447 | ​ |  | 2,933 | ​ |  | (5) | ​ |  | 93,375 | ​ |  | 93,375 | ​
Total investment securities | ​ | $ | 3,376,070 | ​ | $ | 19,467 | ​ | $ | (16,614) | ​ | $ | 3,378,923 | ​ | $ | 3,378,923 | ​
​
(1) | Included in the carrying value of residential mortgage- backed securities are $571,247 of mortgage-backed securities issued by Ginnie Mae and $2,714,301 of mortgage-backed securities issued by Fannie Mae and Freddie Mac
The amortized cost and estimated fair value of investment securities at December 31, 2019, by contractual maturity, are shown below. Expected maturities will differ from contractual maturities because borrowers may have the right to prepay obligations with or without prepayment penalties. ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | 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 | ​ |  | — | ​ |  | — | ​ |  | 2,241 | ​ |  | 2,249 | ​
Due after ten years | ​ |  | — | ​ |  | — | ​ |  | 88,206 | ​ |  | 91,126 | ​
Residential mortgage-backed securities | ​ |  | — | ​ |  | — | ​ |  | 3,285,623 | ​ |  | 3,285,548 | ​
Total investment securities | ​ | $ | 2,400 | ​ | $ | 2,400 | ​ | $ | 3,376,070 | ​ | $ | 3,378,923 | ​
​ 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, $2,721,717 of mortgage-backed securities issued by Fannie Mae and Freddie Mac
Residential mortgage-backed securities are securities issued by Freddie Mac, Fannie Mae, Ginnie Mae or non-government entities. Investments in residential mortgage-backed securities issued by Ginnie Mae are fully guaranteed by the U.S. government. Investments in mortgage-backed securities issued by Freddie Mac and Fannie Mae are not fully guaranteed by the U.S. government; however, we believe that the quality of the bonds is similar to other AAA rated bonds with limited credit risk, particularly given the placement of Fannie Mae and Freddie Mac into conservatorship by the federal government in early September 2008 and because securities issued by others that are collateralized by residential mortgage-backed securities issued by Fannie Mae and Freddie Mac are rated consistently as AAA rated securities. The amortized cost and fair value of available for sale investment securities pledged to qualify for fiduciary powers, to secure public monies as required by law, repurchase agreements and short-term fixed borrowings was $856,135,000 and $855,141,000, respectively, at December 31, 2019. Proceeds from the sale and call of securities available-for-sale were $94,585,000, $59,782,000 and $396,066,000 during 2019, 2018 and 2017, respectively, which amounts included $0, $0 and $377,756,000 of mortgage-backed securities. Gross gains of $3,000, $3,000 and $1,186,000, and gross losses of $15,000, $144,000 and $5,960,000 were realized on the sales in 2019, 2018 and 2017, respectively. 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, 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 |  | $ | 523,031 |  | $ | (2,269) |  | $ | 1,448,109 |  | $ | (14,340) |  | $ | 1,971,140 |  | $ | (16,609) | ​
Obligations of states and political subdivisions | ​ |  | 766 | ​ |  | (5) | ​ |  | — | ​ |  | — | ​ |  | 766 | ​ |  | (5) | ​
​ | ​ | $ | 523,797 | ​ | $ | (2,274) | ​ | $ | 1,448,109 | ​ | $ | (14,340) | ​ | $ | 1,971,906 | ​ | $ | (16,614) | ​
​ 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, 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) | ​
​ The unrealized losses on investments in residential mortgage-backed securities are primarily caused by changes in market interest rates. Residential mortgage-backed securities are primarily securities issued by Freddie Mac, Fannie Mae and Ginnie Mae. The contractual cash obligations of the securities issued by Ginnie Mae are fully guaranteed by the U.S. government. The contractual cash obligations of the securities issued by Freddie Mac and Fannie Mae are not fully guaranteed by the U.S. government; however, we believe that the quality of the bonds is similar to other AAA rated bonds with limited credit risk, particularly given the placement of Fannie Mae and Freddie Mac into conservatorship by the federal government in early September 2008 and because securities issued by others that are collateralized by residential mortgage-backed securities issued by Fannie Mae and Freddie Mac are rated consistently as AAA rated securities. The decrease in fair value on residential mortgage-backed securities issued by Freddie Mac, Fannie Mae and Ginnie Mae is due to market interest rates. We have no intent to sell and more likely than not be required to sell before a market price recovery or maturity of the securities; therefore, it is our conclusion that the investments in residential mortgage-backed securities issued by Freddie Mac, Fannie Mae and Ginnie Mae are not considered other-than-temporarily impaired. The unrealized losses on investments in other securities are caused by fluctuations in market interest rates. The underlying cash obligations of the securities are guaranteed by the entity underwriting the debt instrument. We believe that the entity issuing the debt will honor its interest payment schedule, as well as the full debt at maturity. We purchased the securities for their economic value. The decrease in fair value is primarily due to market interest rates and not other factors, and because we have no intent to sell and will more likely than not be required to sell before a market price recovery or maturity of the securities, it is our conclusion that the investments are not considered other-than-temporarily impaired. Equity securities with readily determinable fair values consist primarily of Community Reinvestment Act funds. At December 31, 2019 and December 31, 2018, the balance in equity securities with readily determinable fair values recorded at fair value were $6,095,000 and $5,937,000, respectively. Prior to January 1, 2018, the equity securities were included in available-for-sale securities, with the related unrealized gain or loss recorded as a component of other comprehensive income (loss). The following is a summary of unrealized and realized gains and losses recognized in net income on equity securities during the twelve months ended December 31, 2019 and December 31, 2018: ​
​ | ​ | ​ | ​ | ​
​ | ​ | Year Ended
​ | ​ | December 31, 2019
​ | ​ | (Dollars in Thousands)
​ | ​ | ​
Net gains recognized during the period on equity securities |  | $ | ​ | 158
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 | ​ | $ | ​ | 158
​ | ​ | ​ | ​ | ​
​
​ | ​ | ​ | ​ | ​
​ | ​ | Year Ended
​ | ​ | December 31, 2018
​ | ​ | (Dollars in Thousands)
​ | ​ | ​
Net losses recognized during the period on equity securities |  | $ | ​ | (388)
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 | ​ | $ | ​ | (388)
​

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- Definition The entire disclosure for investments in certain debt and equity securities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -URI http://asc.fasb.org/subtopic&trid=2209399 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -URI http://asc.fasb.org/subtopic&trid=2324412 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -URI http://asc.fasb.org/topic&trid=2196928 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 940 -SubTopic 320 -URI http://asc.fasb.org/subtopic&trid=2176304 + Details
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