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Investments | 12 Months Ended
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Dec. 31, 2017
Investments [Abstract] |
Investments | INVESTMENTS 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 |
Investments in mortgage-backed securities primarily represent securities issued by the Government National Mortgage Association, Federal National Mortgage Association and Federal Home Loan Mortgage Corporation. Investments in equity securities and corporate bonds represent positions in securities of other financial institutions. Other investments include trust preferred securities of financial institutions. 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 |
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 |
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 |
Investment securities with an aggregate fair value of $2.98 billion have had continuous unrealized losses for more than 12 months as of December 31, 2017 with an aggregate unrealized loss of $59.5 million. As of December 31, 2017, 227 of these investments are government sponsored enterprise-issued mortgage-backed securities and 4 are U.S. Treasury securities. None of the unrealized losses identified as of December 31, 2017 or December 31, 2016 relate to the marketability of the securities or the issuer’s ability to honor redemption obligations. Rather, the unrealized losses related to changes in interest rates relative to when the investment securities were purchased. For all periods presented, BancShares had the ability and intent to retain these securities for a period of time sufficient to recover all unrealized losses. Therefore, none of the securities were deemed to be other than temporarily impaired. Investment securities having an aggregate carrying value of $4.59 billion at December 31, 2017 and $4.55 billion at December 31, 2016 were pledged as collateral to secure public funds on deposit and certain short-term borrowings, and for other purposes as required by law.

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- Definition The entire disclosure for investments in certain debt and equity securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -URI http://asc.fasb.org/topic&trid=2196928 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 320 -URI http://asc.fasb.org/subtopic&trid=4738149 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 940 -SubTopic 320 -URI http://asc.fasb.org/subtopic&trid=2176304 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -URI http://asc.fasb.org/subtopic&trid=2324412 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -URI http://asc.fasb.org/subtopic&trid=2209399 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 954 -SubTopic 320 -URI http://asc.fasb.org/subtopic&trid=2289146 + Details
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