| **Investment Securities**                              | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|--------------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Investments, Debt and Equity Securities [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
| Investment Securities                                  | Investment SecuritiesOur 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:December 31, 2014        (dollars in thousands) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair ValueAvailable-for-Sale:        Obligations of U.S. government sponsored enterprises $2,028 $16 $— $2,044Residential mortgage-backed securities-GSE 295,300 438 5,593 290,145Residential mortgage-backed securities-Private 16,455 820 4 17,271Commercial mortgage backed securities-GSE 22,377 — 419 21,958Commercial mortgage-backed securities-Private 10,365 — 150 10,215Corporate notes 8,399 8 — 8,407Total available-for-sale $354,924 $1,282 $6,166 $350,040Held-to-Maturity:        Residential mortgage-backed securities-GSE 132,396 116 1,635 130,877Commercial mortgage-backed securities-Private 10,065 — 67 9,998     Total held-to-maturity $142,461 $116 $1,702 $140,875Total investment securities $497,385 $1,398 $7,868 $490,915         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 $— $2,077Business Development Company investment 1,753 984 — 2,737Residential 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 9,585Corporate 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 $4.9 million of FHLB stock at December 31, 2014 and $5.9 million at December 31, 2013.  Due to the redemption provisions of FHLB stock, we estimate that fair value approximates cost and that this investment is not impaired at December 31, 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 December 31, 2014 and December 31, 2013, the Bank owned a total of $4.7 million and $3.1 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 December 31, 2014.  FRBR stock is included in other assets at its original cost basis.These investments recorded at cost are included in Other Assets on the Consolidated Balance Sheets, as follows: (dollars in thousands) December 31,  2014 2013Federal Home Loan Bank stock $4,855 $5,878Federal Reserve Bank stock 4,713 4,264Total other investments $9,568 $10,142Gross gains and losses recognized (by specific identification) on the sale of securities are summarized as follows:(dollars in thousands) Years Ended December 31,  2014 2013 2012Proceeds from sales of investment securities $37,239 $177,826 $212,609       Gains on sales of investment securities available-for-sale 1,392 2,953 4,501Losses on sales of investment securities available-for-sale (418) (181) (380)Total securities gains, net $974 $2,772 $4,121At December 31, 2014, $121.9 million of the investment securities portfolio was pledged to secure public deposits, $16.8 million was pledged to retail repurchase agreements and $173.2 million was pledged to others, principally the FHLB and FRBR, leaving $180.6 million available as unpledged securities collateral. At December 31, 2013, $91.2 million of the investment securities portfolio was pledged to secure public deposits, $18.8 million was pledged to retail repurchase agreement and $77.4 million was pledged to others, leaving $376.3 million available as lendable collateral.The following tables show our investments' estimated fair value and gross unrealized losses, aggregated by investment category and length of time that the individual securities have been in a continuous unrealized loss position, at December 31, 2014 and December 31, 2013.  The change in unrealized losses during the years ending December 31, 2014 and 2013 was attributed to changes in interest rates and not to changes in the credit quality of these securities. All unrealized losses on investment securities are considered by management to be temporary given the credit quality of these investment securities or the short duration of the unrealized loss, or both. Less than 12 Months 12 Months or More Total(dollars in thousands)Estimated Fair ValueGross Unrealized Losses Estimated Fair ValueGross Unrealized Losses Estimated Fair ValueGross Unrealized LossesDecember 31, 2014        Available-for-Sale:        Residential mortgage-backed securities-GSE$—$— $245,457$5,593 $245,457$5,593Residential mortgage-backed securities-Private1,1544 —— 1,1544Commercial mortgage-backed securities-GSE—— 21,958419 21,958419Commercial mortgage-backed securities-Private—— 10,215150 10,215150Total available-for-sale$1,154$4 $277,630$6,162 $278,784$6,166Held-to-Maturity:        Residential mortgage-backed securities-GSE$—$— $112,878$1,635 $112,878$1,635Commercial mortgage-backed securities-Private—— 9,99867 9,99867Total held-to-maturity$—$— $122,876$1,702 $122,876$1,702Total$1,154$4 $400,506$7,864 $401,660$7,868         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 December 31, 2014, there were 33 investment securities that had continuous unrealized losses for more than twelve months. At December 31, 2013, there were 10 investment securities that had continuous unrealized losses for more than twelve months. The unrealized losses relate to fixed-rate debt securities that have incurred fair value reductions due to higher market interest rates and, for certain securities, increased credit spreads since the respective purchase date. The unrealized losses are not likely to reverse unless and until market interest rates and credit spreads decline to the levels that existed when the securities were purchased. Unrealized losses for all investment securities are reviewed to determine whether the losses are other than temporary. Investment securities are evaluated on at least a quarterly basis and more frequently when economic or market conditions warrant such an evaluation to determine whether a decline in their value below amortized cost is other than temporary. In conducting this assessment, COB evaluates a number of factors including, but not limited to:•How much fair value has declined below amortized cost;•How long the decline in fair value has existed;•The financial condition of the issuer;•Contractual or estimated cash flows of the security;•Underlying supporting collateral;•Past events, current conditions, forecasts;•Significant rating agency changes on the issuer; and•COB’s intent and ability to hold the security for a period of time sufficient to allow for any anticipated recovery in fair value.COB analyzed its securities portfolio at December 31, 2014 and 2013, 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 December 31, 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 ValueU.S. government sponsored agencies        Due after one year through five years $2,028 $2,044 $— $—Residential mortgage-backed securities-GSE        Due after five years through 10 years 2,327 2,367 — —Due after ten years 292,973 287,778 132,396 130,877Residential mortgage-backed securities-Private        Due after ten years 16,455 17,271 — —Commercial mortgage-backed securities-GSE        Due after five years through 10 years 22,377 21,958 — —Commercial mortgage-backed securities-Private        Due after ten years 10,365 10,215 10,065 9,998Corporate notes        Due in one year or less 8,399 8,407 — —Total $354,924 $350,040 $142,461 $140,875 |

| X                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_InvestmentsDebtAndEquitySecuritiesAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -Glossary Debt Security -URI http://asc.fasb.org/extlink&oid=6509901Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 30 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15032-111544Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6957658&loc=d3e62557-112803Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563Reference 8: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27405-111563Reference 9: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -Glossary Equity Security -URI http://asc.fasb.org/extlink&oid=6511694
+ Details**Name:**us-gaap_InvestmentsInDebtAndMarketableEquitySecuritiesAndCertainTradingAssetsDisclosureTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |