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Investment Securities (Tables) | 12 Months Ended
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Dec. 31, 2018
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 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 Debt Securities |
|  |  |  |  | Gross |  | Gross |  |  |  |  |  |  |
|  | Amortized |  | unrealized |  | unrealized |  | Estimated |  | Carrying |
|  | cost |  | gains |  | losses |  | fair 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 Mac
The amortized cost and estimated fair value by type of investment security at December 31, 2017 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 |
|  |  |  |  | Gross |  | Gross |  | Estimated |  |  |  |
|  | Amortized |  | unrealized |  | unrealized |  | fair |  | Carrying |
|  | cost |  | gains |  | losses |  | value |  | value(1) |
|  | (Dollars in Thousands) |
Residential mortgage-backed securities |  | $ | 3,943,092 |  | $ | 14,110 |  | $ | (65,969) |  | $ | 3,891,233 |  | $ | 3,891,233 |
Obligations of states and political subdivisions |  |  | 225,096 |  |  | 7,871 |  |  | (16) |  |  | 232,951 |  |  | 232,951 |
Equity securities |  |  | 28,075 |  |  | 293 |  |  | (482) |  |  | 27,886 |  |  | 27,886 |
Total investment securities |  | $ | 4,196,263 |  | $ | 22,274 |  | $ | (66,467) |  | $ | 4,152,070 |  | $ | 4,152,070 |
| (1) |  | Included in the carrying value of residential mortgage‑ backed securities are $654,063 of mortgage‑backed securities issued by Ginnie Mae and $3,237,170 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 |  | $ | — |  | $ | — |  | $ | — |  | $ | — |
Due after one year through five years |  |  | 1,200 |  |  | 1,200 |  |  | — |  |  | — |
Due after five years through ten years |  |  | — |  |  | — |  |  | 500 |  |  | 502 |
Due after ten years |  |  | — |  |  | — |  |  | 185,299 |  |  | 187,838 |
Residential mortgage-backed securities |  |  | — |  |  | — |  |  | 3,295,366 |  |  | 3,223,010 |
Total investment securities |  | $ | 1,200 |  | $ | 1,200 |  | $ | 3,481,165 |  | $ | 3,411,350 |
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, 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) |
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, 2017 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 |  | $ | 1,061,577 |  | $ | (13,157) |  | $ | 2,029,455 |  | $ | (52,812) |  | $ | 3,091,032 |  | $ | (65,969) |
Obligations of states and political subdivisions |  |  | 5,534 |  |  | (9) |  |  | 522 |  |  | (7) |  |  | 6,056 |  |  | (16) |
Equity securities |  |  | 11,499 |  |  | (251) |  |  | 8,019 |  |  | (231) |  |  | 19,518 |  |  | (482) |
|  | $ | 1,078,610 |  | $ | (13,417) |  | $ | 2,037,996 |  | $ | (53,050) |  | $ | 3,116,606 |  | $ | (66,467) |
Reconciliation of credit-related impairment charges on available-for-sale investment | The following table presents a reconciliation of credit‑related impairment charges on available‑for‑sale investments recognized in earnings for the twelve months ended December 31, 2017 (in Thousands):
Balance at December 31, 2016 |  | $ | 13,931
Sale of other-than-temporarily impaired available-for-sale securities during period |  |  | (13,931)
Balance at December 31, 2017 |  | $ | —
The following table presents a reconciliation of credit‑related impairment charges on available‑for‑sale investments recognized in earnings for the twelve months ended December 31, 2016 (in Thousands):
Balance at December 31, 2015 |  | $ | 13,577
Impairment charges recognized during period |  |  | 354
Balance at December 31, 2016 |  | $ | 13,931
Summary of unrealized and realized gains and losses recognized in net income on equity securities |
|  | Year Ended
|  | December 31,
|  | (Dollars in Thousands)
|  | 2018
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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