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Investment Securities (Tables) | 12 Months Ended
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Dec. 31, 2017
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, 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 |  |  |  |  |  |  |
|  | Amortized |  | unrealized |  | unrealized |  | Estimated |  | Carrying |
|  | cost |  | gains |  | losses |  | fair 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
The amortized cost and estimated fair value by type of investment security at December 31, 2016 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 |  | $ | 3,946,144 |  | $ | 18,246 |  | $ | (69,920) |  | $ | 3,894,470 |  | $ | 3,894,470 |
Obligations of states and political subdivisions |  |  | 244,622 |  |  | 10,783 |  |  | (433) |  |  | 254,972 |  |  | 254,972 |
Equity securities |  |  | 28,075 |  |  | 314 |  |  | (482) |  |  | 27,907 |  |  | 27,907 |
Total investment securities |  | $ | 4,218,841 |  | $ | 29,343 |  | $ | (70,835) |  | $ | 4,177,349 |  | $ | 4,177,349 |
| (1) |  | Included in the carrying value of residential mortgage‑ backed securities are $850,033 of mortgage‑backed securities issued by Ginnie Mae, $3,029,832 of mortgage‑backed securities issued by Fannie Mae and Freddie Mac and $17,605 issued by non‑government entities
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 |  | $ | 2,275 |  | $ | 2,275 |  | $ | — |  | $ | — |
Due after one year through five years |  |  | 125 |  |  | 125 |  |  | — |  |  | — |
Due after five years through ten years |  |  | — |  |  | — |  |  | 1,939 |  |  | 1,967 |
Due after ten years |  |  | — |  |  | — |  |  | 223,157 |  |  | 230,984 |
Residential mortgage-backed securities |  |  | — |  |  | — |  |  | 3,943,092 |  |  | 3,891,233 |
Equity securities |  |  | — |  |  | — |  |  | 28,075 |  |  | 27,886 |
Total investment securities |  | $ | 2,400 |  | $ | 2,400 |  | $ | 4,196,263 |  | $ | 4,152,070 |
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, 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) |
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, 2016 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 |  | $ | 2,513,872 |  | $ | (52,245) |  | $ | 396,695 |  | $ | (17,675) |  | $ | 2,910,567 |  | $ | (69,920) |
Obligations of states and political subdivisions |  |  | 31,104 |  |  | (433) |  |  | — |  |  | — |  |  | 31,104 |  |  | (433) |
Equity securities |  |  | 14,066 |  |  | (184) |  |  | 5,452 |  |  | (298) |  |  | 19,518 |  |  | (482) |
|  | $ | 2,559,042 |  | $ | (52,862) |  | $ | 402,147 |  | $ | (17,973) |  | $ | 2,961,189 |  | $ | (70,835) |
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
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, 2015 (in Thousands):
Balance at December 31, 2014 |  | $ | 12,623
Impairment charges recognized during period |  |  | 954
Balance at December 31, 2015 |  | $ | 13,577

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