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Investments (Tables) | 12 Months Ended
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Dec. 31, 2017
Investments [Abstract] |
Aggregate Values And Unrealized Gains And Losses Of Investment Securities | The amortized cost and fair value of investment securities classified as available for sale and held to maturity at December 31, 2017 and 2016, were as follows:
| December 31, 2017
(Dollars in thousands) | Cost |  | Gross unrealized gains |  | Gross unrealized losses |  | Fair value
Investment securities available for sale |  |  |  |  |  |  |
U.S. Treasury | $ | 1,658,410 |  |  | $ | — |  |  | $ | 546 |  |  | $ | 1,657,864 |
Mortgage-backed securities | 5,428,074 |  |  | 1,544 |  |  | 80,192 |  |  | 5,349,426 |
Equity securities | 75,471 |  |  | 29,737 |  |  | — |  |  | 105,208 |
Corporate bonds | 59,414 |  |  | 557 |  |  | 8 |  |  | 59,963 |
Other | 7,645 |  |  | 256 |  |  | 182 |  |  | 7,719 |
Total investment securities available for sale | $ | 7,229,014 |  |  | $ | 32,094 |  |  | $ | 80,928 |  |  | $ | 7,180,180 |
| December 31, 2016
| Cost |  | Gross unrealized gains |  | Gross unrealized losses |  | Fair value
U.S. Treasury | $ | 1,650,675 |  |  | $ | 579 |  |  | $ | 935 |  |  | $ | 1,650,319 |
Government agency | 40,291 |  |  | 107 |  |  | — |  |  | 40,398 |
Mortgage-backed securities | 5,259,466 |  |  | 2,809 |  |  | 86,850 |  |  | 5,175,425 |
Equity securities | 71,873 |  |  | 11,634 |  |  | — |  |  | 83,507 |
Corporate bonds | 49,367 |  |  | 195 |  |  | — |  |  | 49,562 |
Other | 7,615 |  |  | — |  |  | 246 |  |  | 7,369 |
Total investment securities available for sale | $ | 7,079,287 |  |  | $ | 15,324 |  |  | $ | 88,031 |  |  | $ | 7,006,580 |
| December 31, 2017
| Cost |  | Gross unrealized gains |  | Gross unrealized losses |  | Fair value
Investment securities held to maturity |  |  |  |  |  |  |
Mortgage-backed securities | $ | 76 |  |  | $ | 5 |  |  | $ | — |  |  | $ | 81 |
| December 31, 2016
| Cost |  | Gross unrealized gains |  | Gross unrealized losses |  | Fair value
Mortgage-backed securities | $ | 98 |  |  | $ | 6 |  |  | $ | — |  |  | $ | 104 |
Investment Securities Maturity Information | The following table provides the amortized cost and fair value by contractual maturity. Expected maturities will differ from contractual maturities on certain securities because borrowers and issuers may have the right to call or prepay obligations with or without prepayment penalties. Repayments of mortgage-backed securities are dependent on the repayments of the underlying loan balances. Equity securities do not have a stated maturity date.
| December 31, 2017 |  | December 31, 2016
(Dollars in thousands) | Cost |  | Fair value |  | Cost |  | Fair value
Investment securities available for sale |  |  |  |  |  |  |
Non-amortizing securities maturing in: |  |  |  |  |  |  |
One year or less | $ | 808,768 |  |  | $ | 808,301 |  |  | $ | 842,798 |  |  | $ | 842,947 |
One through five years | 849,642 |  |  | 849,563 |  |  | 848,168 |  |  | 847,770 |
Five through 10 years | 59,414 |  |  | 59,963 |  |  | 49,367 |  |  | 49,562 |
Over 10 years | 7,645 |  |  | 7,719 |  |  | 7,615 |  |  | 7,369 |
Mortgage-backed securities | 5,428,074 |  |  | 5,349,426 |  |  | 5,259,466 |  |  | 5,175,425 |
Equity securities | 75,471 |  |  | 105,208 |  |  | 71,873 |  |  | 83,507 |
Total investment securities available for sale | $ | 7,229,014 |  |  | $ | 7,180,180 |  |  | $ | 7,079,287 |  |  | $ | 7,006,580 |
Investment securities held to maturity |  |  |  |  |  |  |
Mortgage-backed securities held to maturity | $ | 76 |  |  | $ | 81 |  |  | $ | 98 |  |  | $ | 104 |
Securities Gains (Losses) | For each period presented, securities gains (losses) include the following:
| Year ended December 31
(Dollars in thousands) | 2017 |  | 2016 |  | 2015
Gross gains on retirement/sales of investment securities available for sale | $ | 11,635 |  |  | $ | 27,104 |  |  | $ | 10,834 |
Gross losses on sales of investment securities available for sale | (7,342 | ) |  | (431 | ) |  | (17 | )
Net securities gains | $ | 4,293 |  |  | $ | 26,673 |  |  | $ | 10,817 |
Investment Securities With Unrealized Losses | The following table provides information regarding securities with unrealized losses as of December 31, 2017 and 2016:
| December 31, 2017
| Less than 12 months |  | 12 months or more |  | Total
(Dollars in thousands) | Fair Value |  | Unrealized Losses |  | Fair Value |  | Unrealized Losses |  | Fair Value |  | Unrealized Losses
Investment securities available for sale: |  |  |  |  |  |  |  |  |  |  |
U.S. Treasury | $ | 1,408,166 |  |  | $ | 345 |  |  | $ | 249,698 |  |  | $ | 201 |  |  | $ | 1,657,864 |  |  | $ | 546 |
Mortgage-backed securities | 2,334,102 |  |  | 20,923 |  |  | 2,725,933 |  |  | 59,269 |  |  | 5,060,035 |  |  | 80,192 |
Corporate bonds | 5,025 |  |  | 8 |  |  | — |  |  | — |  |  | 5,025 |  |  | 8 |
Other | 5,349 |  |  | 182 |  |  | — |  |  | — |  |  | 5,349 |  |  | 182 |
Total | $ | 3,752,642 |  |  | $ | 21,458 |  |  | $ | 2,975,631 |  |  | $ | 59,470 |  |  | $ | 6,728,273 |  |  | $ | 80,928 |
| December 31, 2016
| Less than 12 months |  | 12 months or more |  | Total
| Fair Value |  | Unrealized Losses |  | Fair Value |  | Unrealized Losses |  | Fair Value |  | Unrealized Losses
Investment securities available for sale: |  |  |  |  |  |  |  |  |  |  |
U.S. Treasury | $ | 807,822 |  |  | $ | 935 |  |  | $ | — |  |  | $ | — |  |  | $ | 807,822 |  |  | $ | 935 |
Mortgage-backed securities | 4,442,700 |  |  | 82,161 |  |  | 362,351 |  |  | 4,689 |  |  | 4,805,051 |  |  | 86,850 |
Other | 7,369 |  |  | 246 |  |  | — |  |  | — |  |  | 7,369 |  |  | 246 |
Total | $ | 5,257,891 |  |  | $ | 83,342 |  |  | $ | 362,351 |  |  | $ | 4,689 |  |  | $ | 5,620,242 |  |  | $ | 88,031 |

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