| **Investment Securities**                              | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                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| **Investments, Debt and Equity Securities [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                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| Investment Securities                                  | Investment SecuritiesSecurities designated as available-for-sale are carried at fair value.  However, the unrealized difference between amortized cost and fair value of securities available-for-sale is excluded from net income unless there is an other than temporary impairment and is reported, net of deferred taxes, as a component of shareholders' equity as accumulated other comprehensive income (loss).  Securities designated as held-to-maturity are carried at amortized cost, as the Bank has the ability, and management has the positive intent, to hold these securities to maturity.  Premiums and discounts on securities are amortized and accreted according to the interest method.Our primary objective in managing the investment portfolio is to maintain a portfolio of high quality, highly liquid investments yielding competitive returns.  We are required under federal regulations to maintain adequate liquidity to ensure safe and sound operations.  We maintain investment balances based on a continuing assessment of cash flows, the level of loan production, current interest rate risk strategies and an assessment of the potential future direction of market interest rate changes.  Investment securities differ in terms of default, interest rate, liquidity and expected rate of return risks.The following table summarizes the amortized cost and estimated fair value of investment securities and presents the related gross unrealized gains and losses:June 30, 2014        (dollars in thousands) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair ValueAvailable-for-Sale:        Obligations of:        U.S. government sponsored enterprises $2,040 $24 $— $2,064Residential mortgage-backed securities-GSE 347,916 1,278 12,298 336,896Residential mortgage-backed securities-Private 18,404 1,104 — 19,508Commercial mortgage-backed securities-GSE 22,573 — 522 22,051Commercial mortgage-backed securities-Private 10,387 — 241 10,146Corporate notes 8,395 50 — 8,445Total available-for-sale $409,715 $2,456 $13,061 $399,110Held-to-Maturity:        Residential mortgage-backed securities-GSE 136,987 — 4,589 132,398Commercial mortgage-backed securities-Private 10,068 — 178 9,890     Total held-to-maturity $147,055 $— $4,767 $142,288Total investment securities $556,770 $2,456 $17,828 $541,398         December 31, 2013        (dollars in thousands) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair ValueAvailable-for-Sale:        Obligations of:        U.S. government sponsored enterprises $2,051 $26 $— $2,077Residential mortgage-backed securities-GSE 364,513 974 24,340 341,147Residential mortgage-backed securities-Private 19,770 982 — 20,752Commercial mortgage-backed securities-GSE 22,767 — 1,328 21,439Commercial mortgage-backed securities-Private 10,408 — 823 9,585Business Development Company investment 1,753 984   2,737Corporate notes 16,795 82 — 16,877Total available-for-sale $438,057 $3,048 $26,491 $414,614Held-to-Maturity:        Residential mortgage-backed securities-GSE 141,724 — 9,907 131,817Commercial mortgage-backed securities-Private 10,071 — 763 9,308     Total held-to-maturity $151,795 $— $10,670 $141,125Total investment securities $589,852 $3,048 $37,161 $555,739As a member of the Federal Home Loan Bank of Atlanta (“FHLB”), the Bank is required to own capital stock in the FHLB based generally upon the balances of total assets and FHLB advances.  FHLB capital stock is pledged to secure FHLB advances.  This investment is carried at cost since no ready market exists for FHLB stock and there is no quoted market value.  However, redemption of this stock has historically been at par value.  The Bank owned a total of $5.1 million of FHLB stock at June 30, 2014 and $5.9 million at December 31, 2013.  Due to the redemption provisions of FHLB stock, we have estimated that fair value approximated cost and that this investment was not impaired at June 30, 2014.  FHLB stock is included in other assets at its original cost basis.As a member bank of the Federal Reserve Bank of Richmond (“FRBR”), the Bank also is required to own capital stock of the FRBR based upon a percentage of the Bank's common stock and surplus.  This investment is carried at cost since no ready market exists for FRBR stock and there is no quoted market value.  At June 30, 2014 and December 31, 2013, the Bank owned a total of $4.4 million and $4.3 million of FRBR stock, respectively.  Due to the nature of this investment in an entity of the U.S. government, we have estimated that fair value approximated the cost and that this investment was not impaired at June 30, 2014.  FRBR stock is included in other assets at its original cost basis.At June 30, 2014, $98.2 million of the investment securities portfolio was pledged to secure public deposits, $17.9 million was pledged to retail repurchase agreements and $75.7 million was pledged to others, leaving $354.3 million available as lendable collateral.During the three months and six months ended June 30, 2014, the Bank sold securities with a book value of $2.5 million and recognized a gain of $0.7 million. During the three months and six months ended June 30, 2013, the Bank sold securities with a book value of $25.7 million and $171.8 million, respectively, and recognized gross gains of $0.3 million and $2.9 million, respectively, and gross losses of $0 and $0.2 million, respectively. The Bank sold securities in 2014 after an SBIC Fund investment converted to a publicly traded equity.  The Bank sold securities in 2013 in order to manage our interest rate sensitivity profile. The following tables show our investments' estimated fair value and gross unrealized losses, aggregated by investment category and length of time that the individual securities have been in a continuous unrealized loss position, at June 30, 2014 and December 31, 2013.  The change in unrealized losses during the six months ending June 30, 2014 was attributed to changes in interest rates and not to changes in the credit quality of these securities. All unrealized losses on investment securities are considered by management to be temporary given the credit quality of these investment securities or the short duration of the unrealized loss, or both. Less than 12 Months 12 Months or More Total(dollars in thousands)Estimated Fair ValueGross Unrealized Losses Estimated Fair ValueGross Unrealized Losses Estimated Fair ValueGross Unrealized LossesJune 30, 2014        Available-for-Sale:        Residential mortgage-backed securities-GSE$14$— $300,334$12,298 $300,348$12,298Commercial mortgage-backed securities-GSE—— 22,052522 22,052522Commercial mortgage-backed securities-Private—— 10,146241 10,146241Total available-for-sale$14$— $332,532$13,061 $332,546$13,061Held-to-Maturity:        Residential mortgage-backed securities-GSE$—$— $132,398$4,589 $132,398$4,589Commercial mortgage-backed securities-Private—— 9,890178 9,890178Total held-to-maturity$—$— $142,288$4,767 $142,288$4,767Total$14$— $474,820$17,828 $474,834$17,828         December 31, 2013        Available-for-Sale        Residential mortgage-backed securities-GSE$222,475$16,585 $76,501$7,755 $298,976$24,340Commercial mortgage-backed securities-GSE—— 21,4391,328 21,4391,328Commercial mortgage-backed securities-Private9,585823 —— 9,585823Total available-for-sale$232,060$17,408 $97,940$9,083 $330,000$26,491Held-to-Maturity:        Residential mortgage-backed securities-GSE$131,817$9,907 $—$— $131,817$9,907Commercial mortgage-backed securities-Private9,308763 —— 9,308763Total held-to-maturity$141,125$10,670 $—$— $141,125$10,670Total$373,185$28,078 $97,940$9,083$471,125$37,161At June 30, 2014 and December 31, 2013, there were thirty-eight and ten available-for-sale securities that were in an unrealized loss position for 12 months or more, respectively.COB analyzed its securities portfolio at June 30, 2014, and considered ratings, fair value, cash flows and other factors to determine if any of the securities were other than temporarily impaired. Since none of the unrealized losses relate to the marketability of the securities or the issuer’s ability to honor redemption obligations, and COB has determined that it is not more likely than not that COB will be required to sell the security before recovery of its amortized cost basis, none of the securities are deemed to be other than temporarily impaired.The aggregate amortized cost and fair value of securities at June 30, 2014, by remaining contractual maturity, are shown in the following table.  Actual expected maturities may differ from contractual maturities because issuers may have the right to call or prepay obligations.  Available-for-Sale Held-to-Maturity(dollars in thousands) Amortized Cost Estimated Fair Value Amortized Cost Estimated Fair ValueDue in one year or less $8,395 $8,445 $— $—Due after one year through five years 2,040 2,064 — —Total 10,435 10,509 — —Mortgage-backed securities 399,280 388,601 147,055 142,288Total $409,715 $399,110 $147,055 $142,288 |

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| - Definition
The entire disclosure for  investments in certain debt and equity securities.
+ References
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